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Compound
Stoichiometry
Chemistry
Unit 5
Nov. 7-16, 2012
In this unit, we shall begin the study of stoichiometry (from the Greek words "stoicheion" which
means element and "metron" which means to measure. Stoichiometry problems are those
problems which involve calculations. There are 2 types of stoichiometry problems – those
requiring a balanced chemical equation (which we do not know how to write yet) and those
problems which do not require a balanced equation, but do required a correctly-written formula.
Thus, we have COMPOUND STOICHIOMETRY which we are just about to learn and
REACTION STOICHIOMETRY which we will learn in about 3 weeks.
There are several different types of problems that you need to be able to recognize and calculate.
1. FORMULA or MOLECULAR MASS – these are two different phrases that really mean the
same thing mathematically. We use the term formula mass for ionic compounds and the term
molecular mass for molecular compounds (bonded covalently).
Step 1: Write a correct formula for the compound
Step 2: List the elements separately
Step 3: Beside the element’s symbol, write the number of atoms of each
Step 4: Multiple by the average atomic mass given on the Periodic Table. Round each
mass to two decimal places.
Step 5: Calculate the answer for each
Step 6: Add the answers
Example 1: Find the formula mass of calcium phosphate.
Example 2: Find the formula mass of ammonium sulfate
Example 3: Find the molecular mass of dichlorine heptoxide.
Hydrates are ionic compounds that trap water molecules in their crystalline structures. You do
not need to worry about which ionic compounds will do this and which will not – you just need
to know how to name and recognize hydrates. A hydrate will be written as a correct ionic
formula followed by a dot (that looks like a multiplication dot) and then some number of
molecules of water: CuSO45H2O
To name a hydrate, simply name the ionic part as usual. For example if you are asked to name
the compound CuSO45H2O, you will name the ionic part normally – copper(II) sulfate. Then
you will add another word that consists of a prefix that indicates how many water molecules are
trapped and the word "hydrate". Thus, the above-mentioned compound is copper(II) sulfate
pentahydrate. We will use the same prefixes as you learned for the molecular compounds.
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Prefixes for hydrates:
1 – mono
5 – penta
9 – nona
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2 – di
6 – hexa
10 – deca
3 – tri
7 – hepta
4 – tetra
8 – octa
If you need to find the formula mass of a hydrate, add all of the average atomic masses of the
ionic compound together and then add the mass of "x" number of water molecules to it. The dot
does not mean multiplication.
Example 4: Find the formula mass of barium chlorate hexahydrate.
5A - FORMULA MASS AND MOLECULAR MASS WORKSHEET
Find the formula mass for each of these compounds which are ionic; find
the molecular mass for each of the compounds which are molecular:
1. dichlorine heptoxide
2. cesium sulfite
3. tin (IV) hydroxide
4. acetic acid
5. lead (IV) oxalate
6. magnesium nitrite octahydrate
7. potassium phosphide
8. mercury (II) chloride
9. barium bicarbonate
10. ammonium chromate tetrahydrate
11. oxygen gas
12. strontium cyanide
13. silver perchlorate
14. iron (II) phosphate
15. nitric acid
16. copper (I) nitrate
17. cadmium dichromate decahydrate
18. zinc nitride
19. diphosphorus pentoxide
20. copper (II) bromide
2. MOLE PROBLEMS – The term mole is a unit used in chemistry that indicates a definite
number of particles. In the American system of measures, we know that a "dozen" is a
measurement that always means 12, no matter what we are measuring: eggs, pencils, or
doughnuts.
The mole is a unit that means the number of representative particles found in the
formula/molecular mass of a substance expressed in grams.
The representative particle is important. For elements (except diatomics), it is the
atom; for compounds and diatomics, it is the molecule (molecular compounds or
diatomics) or the formula unit (ionic)
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1 mole of sodium chloride is a different number of grams than 1 mole of water or 1
mole of calcium carbonate, but one mole of each of the compounds contains the
same number of representative particles.
Sodium chloride weighs 58.5 amu; therefore 58.5 grams = 1 mole of NaCl
Water weighs 18 amu; therefore, there are 18 grams of water in 1 mole of water.
Calcium carbonate has a formula mass of 100 amu. This means that it takes 100
grams of calcium carbonate to make 1 mole of calcium carbonate.
Another label for formula or molecular mass can be g/mol – grams per mole (a good
conversion factor since it is a double label).
Example 5: What if I have 200 grams of calcium carbonate, how many moles is this?
You calculate mole problems using dimensional analysis and the formula mass as a
conversion factor.
Example 6: How many moles are there in 28.7 grams of lithium nitrate
Example 7: How many moles are there in 100 grams of nitrogen gas?
Be careful of the molecular mass of diatomics!!!!!!
Example 8: If 47.4 grams of an ionic compound are known to be 1.185 moles of that
compound, what is the formula mass of the compound?
Example 9: If 56 grams of a pure monatomic element are known to be 8.26 moles of that
element, what element are we talking about?
Example 10: If I tell you to measure out 1.34 moles of water, how many grams of water is
this?
5B - MOLES WORKSHEET
1. How many moles are there in 768 grams of iron(III) sulfate?
2. How many moles are there in 1.2 kg of potassium dichromate?
3. I have 13.2 moles of lead(II) nitrate. How many grams is this?
4. How many pounds would 5.75 moles of magnesium hydroxide weigh?
5. How many moles are there in 78 grams of nitrogen?
6. How many grams would 12.30 moles of sulfur hexafluoride weigh?
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7. I have 0.51101 pounds of an unknown compound, and it is known this
amount represents 4.50 moles of this compound. What is the molecular
mass of the compound?
8. I have 7.60 moles of chlorine gas. Would this amount weigh more than a
pound? Prove your answer.
9. What is the molecular mass of a compound if it is known that 658 grams
of this compound represent 6.89 moles?
10. If it is known that 41.6 grams of pure element that is not diatomic
represent 1.30 moles of this element, what is the atomic mass of the
element? What element is it?
11. If the above element were known to be one of the diatomic elements,
what element would you predict it to be?
12. How many moles are there in 34.5 grams of strontium chloride?
13. If the molecular mass of a substance is 203 g/mol, how many moles are
there in 4567 milligrams of this substance?
14. What is the molecular mass of a compound if 0.544 moles weighs
3.4509 grams?
15. What is the molecular mass of a compound if it is known that 8.00 x 10 8
micrograms of it represent 2.55 moles?
16. Which would weigh more: 3.40 moles of aluminum nitrite or 3.40 moles
of cadmium phosphate?
3. AVOGADRO'S NUMBER PROBLEMS – Avogadro's number (6.022 x 1023) is the number
of representative particles in one mole of a substance. For elements (except diatomics), there are
6.022 x 1023 atoms in one mole of that element. For molecules, ionic compounds and diatomics,
there are 6.022 x 1023 molecules or formula units in one mole of that compound.
There are 65 grams in 1 mole of pure zinc metal. Therefore, there are 6.022 x 1023 atoms
of zinc in 65 grams (or 1 mole) of zinc.
There are 18 grams of water in 1 mole of water. Therefore, there are 6.022 x 1023
molecules of water in 18 grams or 1 mole of water.
There are 32 grams of oxygen in 1 mole of oxygen gas. Therefore, there are 6.022 x 1023
diatomic oxygen molecules in 32 grams of oxygen gas or 1 mole of oxygen gas.
However, there are 2(6.022 x 1023) or 1.2044 x 1024 oxygen atoms in 32 grams of
oxygen gas.
There are 58.5 grams of NaCl in 1 mole of this ionic compound. Therefore, there are
6.022 x 1023 formula units of NaCl in 58.5 grams of NaCl.
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Formula unit : ionic compound : : molecule : molecular compound
The most basic or the representative particle of an element (except diatomics) that will
retain the properties of that element is the atom.
The most basic or the representative particle of a molecular (2 non-metals) compound
is the molecule.
The most basic or the representative particle of an ionic compound is called the
formula unit.
Example 11: How many atoms are present in a pure iron nail that weighs 13.2 grams?
Example 12: If there are known to be 8.24 x 1026 molecules of water present, would it weigh
more than a pound?
Example 13: How many moles of carbon dioxide are present in 4.55 x 1024 molecules of
CO2?
WHEN IN DOUBT, GO TO MOLES!!
Example 14: How many formula units are there in 7.55 grams of sodium sulfate?
Example 15: If I know that a sample of mercury(II) chloride contains 7.32 x 1024 formula
units, how much does the sample weigh in pounds?
Example 16: Which would contain more representative particles: 46 grams of sodium
hydroxide or 46 grams of carbon dioxide?
Example 17: Which weighs more: 6.24 x 10
1024 molecules of phosphorus tribromide?
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formula units of sodium bromide or 6.24 x
Being aware of what the representative particle of any substance is very important! The
following problems sound exactly like the ones we have just worked, but they are worked with
an extra step that is essential due to the representative particle being different than the particle
asked for or given in the problem.
Example 18: How many carbon atoms are there in 16.0 grams of glucose?
Example 19: If a sample of aluminum sulfite is known to contain 4.55 x 1023 atoms of
aluminum, how much does the entire sample of aluminum sulfite weigh in pounds?
Example 20: How many nitrogen atoms are there in 45.8 grams of ammonium nitride?
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5C - AVOGADRO'S NUMBER WORKSHEET
1. How many formula units would there be in a 54.3 gram sample of
sodium nitrate?
2. How many atoms of copper are there in 1.00 kg of pure copper?
3. How much would 3.42 x 1023 formula units of mercury(II) oxide weigh in
grams?
4. Which would weigh more: 6.54 x 1022 formula units of aluminum chloride
or 7.34 x 1022 formula units of tin(II) fluoride?
5. How many molecules are present in 0.00115 grams of carbon dioxide?
6. How many atoms of zinc are required to weigh 5.40 ng?
7. I have 3.45 x 1024 molecules of water. How much would this weigh in
pounds? How many moles are there in this amount of water?
8. How many moles are represented by 8.27 x 1023 molecules of any
molecular substance?
9. Which would be more moles of a molecular compound: 6.57 x 10 23
molecules or 6.59 x 1023 molecules. How do you know?
10. How many diatomic molecules of chlorine were there be in 28.0 grams
of chlorine gas? How many atoms of chlorine would be present in this
same amount?
11. How many carbon atoms would there be in 200 grams of aluminum
carbonate?
12. If there are known to be 7.93 x 1024 atoms of oxygen present in a
sample of magnesium sulfate, how much does the sample of magnesium
sulfate weigh in pounds?
13. How many oxygen atoms are there in 2.4 pounds of calcium acetate?
14. How many chlorine atoms are there in 45 grams of mercury(I)
chlorate?
15. If there are known to be 1.06 x 1024 oxygen atoms in a sample of
barium permanganate, what is the mass of the entire sample of barium
permanganate in grams?
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16. If there are known to be 9.76 x 1025 chlorine atoms in a sample of
aluminum chloride, would the entire sample weigh more than 15 pounds?
17. How many oxygen atoms are there in 170 grams of cesium phosphite?
4. PERCENTAGE COMPOSITION PROBLEMS – just like any percentage problems you
are comparing the part to the whole. In chemistry, percentage composition is based on mass, not
on numbers of atoms present. For example, if I have 5 pieces of fruit in a dish and 2 of the pieces
of fruit are bananas, what percentage of the fruit in the dish is bananas?
We are going to find the percentage by mass of certain elements in a compound by comparing
the mass of the element to the mass of the entire compound.
How to find percentage composition:
Step 1: Write a correct formula for the compound in the question
Step2: Find the formula or molecular mass of the entire compound
Step 3: Divide the total mass of the element by the total mass of the entire compound
Step 4: Multiply the result by 100 to convert it to a percentage.
Step 5: Since there are no significant digits given in the problem except masses that have
already been rounded, express your answer to 2 decimal places and place a % sign as the
label.
Example 21: What is the complete percentage composition (by mass) of potassium
dichromate?
Example 22: What is the percentage of calcium in calcium phosphide?
Example 23: What is the percentage of oxygen in barium chlorate hexahydrate?
Remember that a percentage is an excellent conversion factor since it represents a
comparison of units to 100 units. The following problems can be worked using dimensional
analysis or by finding the percentage of a particular element present and then multiplying it times
the mass of the compound given.
Example 24: How many grams of pure magnesium could be recovered from the
decomposition of 49.4 grams of magnesium fluoride?
Example 25: How many grams of calcium are present in 156.8 grams of chalk (calcium
carbonate)?
Example 26: How many grams of pure iron can be recovered from 50 grams of an ore that is
known to be 32% iron(II) nitrate?
Example 27: Limestone is known to be 95% (by mass) calcium carbonate. How many grams
of pure calcium could you except to obtain from a piece of limestone that weighs 100 kg?
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These problems are also percentage composition problems even though they might not
sound like it.
Example 28: Your eccentric uncle has just left you 100 kilograms of brass in his will. Brass
is known to be 72% copper and 28% zinc. Pure copper sells for $2.43 per pound. How
much money will you have if you chemically separate the brass into copper and zinc and sell
just the pure copper?
Example 29: Randall students need money!! They have decided to break into the chemical
storeroom and steal all of the silver nitrate we have, decompose it into its elements and sell
the pure silver for the current price of $7.81/Troy ounce. (1 Troy ounce (ozt) = 31.10
grams). How much money will they have if they decompose 450 grams of silver nitrate?
Example 30: The Science Bowl Team needs money for airplane tickets to go to the national
contest since they have won state. The current price of pure gold is $272/Troy ounce (1 Troy
ounce = 31.10 grams). We have some old gold(III) chloride in the storeroom that is not being
used for anything. How many grams of gold(III) chloride must we decompose and sell the
pure gold out of it in order to raise the $6000 necessary for tickets?
Example 31: The officers of the Key Club have met and decided that, as a fund-raiser, they
would spend $100 to buy 1.0 pound of silver nitrate. They plan to decompose the silver
nitrate and sell the pure silver for $7.81/Troy ounce. How much of a profit will they make on
this fundraiser?
5D - PERCENTAGE COMPOSITION WORKSHEET
1. Calculate the complete percentage composition (by mass) of the
following:
(a) iron (III) oxide
(b) barium phosphate
2. What is the percentage of sodium (by mass) in sodium phosphate?
3. For the hydrate sodium sulfate decahydrate, calculate the following:
(a) the percent of sodium (by mass) in the hydrate
(b) percent of total oxygen (by mass) in the hydrated compound
(c) percent of water (by mass) in the hydrate
4. Calculate the percentage of nitrogen (by mass) in ammonium nitride
5. Calculate the number of grams of magnesium in 50.0 g of magnesium
sulfide
6. Calculate the number of grams of iron in 80.0 kg of iron(II) carbonate
7. Calculate the mass of aluminum (in kg) in 200 kg of aluminum oxide
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8. Calcium chloride can exist as the anhydrous compound CaCl2 or as
three different hydrated forms that are mono-, di-, and hexahydrates.
Calculate the percentage of calcium in each of the different forms of
calcium chloride:
(a) anhydrous calcium chloride
(b) calcium chloride monohydrate
(c) calcium chloride dihydrate
(d) calcium chloride hexahydrate
9. What mass of LEAD could be obtained from 50.0 grams of lead(II) oxide
ore that is only 30.0% pure lead (II) oxide?
10. What mass of iron could be obtained from 40.0 grams of iron(III) oxide
ORE that is only 75.00% pure iron (III) oxide?
11. Water can be decomposed by electricity into hydrogen gas and
oxygen gas. If electricity is passed through 20.0 g water for 4.0 minutes,
and 16.8 grams of water still remain, what percentage of the water has
decomposed?
12. An underground mining company needs to produce 40.0 tons of PURE
silver every month in order to stay in business. If the ORE from the mine
averages 2.00% pure silver, how many tons of silver ORE must be mined
every month in order to obtain the 40.0 tons of silver the company needs
to stay in business?
13. A sample of brass contains (by mass) 28.0% zinc and 72.0% copper. If
you are a brass manufacturer and only have 65.0 kg of copper and an
unlimited supply of zinc, how many kg of brass could you produce?
5. EMPIRICAL FORMULA PROBLEMS – an empirical formula is one that shows the ratio
of atoms in a compound. It is reduced to its very lowest terms, but it does not necessarily give
the exact number of atoms of everything present, but rather the ratio of the atoms present (i.e.,
CH2O is an empirical formula that tells us that there are twice as many hydrogen atoms present
as there are carbon atoms or oxygen atoms.
C6H12O6 is a molecular formula that could be reduced to lower terms, but it isn’t. It tells
us that there are 6 atoms of carbon present, 12 atoms of hydrogen present and 6 atoms of
oxygen present – it gives the numbers of atoms of each and not the simplest ratio between
the atoms)
To solve empirical formula problems, you must be given either the grams of each element
present or the percentage of each element present.
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How to solve empirical formula problems:
Step 1: Divide the grams of each element given by the atomic mass of that element. Be
sure to leave your answer in at least 4 significant figures to maintain your
accuracy. If percentages are given, you can change the percentage to grams since
you would have that many grams in 100g of the total.
Step 2: Compare the results you got in #1 by selecting the smallest of the results and
dividing all of the results by this smallest value. Here you should get a result that
is either very close to a whole number or very close to a mixed number that
contains a recognizable decimal fraction (i.e., 0.333 is 1/3; 0.25 is 1/4; 0.667 is
2/3; 0.50 is 1/2)
Step 3: If all of the results in #2 are whole numbers, simply use these whole numbers as
subscripts and write the empirical formula. If all of the results in #2 are not whole
numbers (i.e., some of them are decimal fraction mixed numbers), you must
multiply all of the results in #2 by some number that will make everything a
whole number. If you have a number of 1.5, multiply all of the results by 2; if you
have a number of 2.25, multiply all of the results by 4. If you have a number of
2.333, multiply all of the numbers by 3.
Step 4: Write the metal first (if there is one); if the compound contains carbon list the
elements in the order C, H, O, N and then alphabetically.
Example 32: A compound is found to contain 51.18% zinc, 11.024% nitrogen and 37.795%
oxygen. What is the empirical formula of the compound?
Example 33: A 200 -gram sample of compound that contains only carbon, hydrogen and
oxygen is found to contain 94.74 grams of carbon, 21.05 grams of hydrogen and 84.21 grams
of oxygen. What is the empirical formula of the compound?
Example 34: An organic compound is analyzed and found to contain 44.226% carbon,
6.388% hydrogen, 23.587% oxygen, and 25.799% nitrogen. What is the empirical formula
for the compound?
5E - EMPIRICAL FORMULA WORKSHEET
1. What is the empirical formula for a compound which contains 0.0134 g
of iron, 0.00769 g of sulfur and 0.0115 g of oxygen?
2. Find the empirical formula for a compound which contains 32.8%
chromium and 67.2% chlorine.
3. NAME the compound which contains 0.463 g Tl, 0.0544 g of carbon,
0.00685 g of hydrogen and 0.0725 g oxygen by finding its empirical
formula.
4. What is the empirical formula for a compound which contains 67.1%
zinc and the rest is oxygen?
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5. Barry Um has a sample of a compound which weighs 200 grams and
contains only carbon, hydrogen, oxygen and nitrogen. By analysis, he
finds that it contains 97.56 grams of carbon, 4.878 g of hydrogen, 52.03 g
of oxygen and 45.53 g of nitrogen. Find its empirical formula.
6. The characteristic odor of pineapple is due to ethyl butyrate, an organic
compound which contains only carbon, hydrogen and oxygen. If a sample
of ethyl butyrate is known to contain 0.62069 g of carbon, 0.103448 g of
hydrogen and 0.275862 g of oxygen, what is the empirical formula for
ethyl butyrate?
7. 300 grams of a compound which contains only carbon, hydrogen and
oxygen is analyzed and found to contain the exact same percentage of
carbon as it has oxygen. The percentage of hydrogen is known to be
5.98823%. Find the empirical formula of the compound.
8. 200.00 grams of an organic compound is known to contain 83.884
grams of carbon, 10.486 grams of hydrogen, 18.640 grams of oxygen and
the rest is nitrogen. What is the empirical formula of the compound?
9. 300 grams of an organic sample which contains only carbon, hydrogen
and oxygen is analyzed and found to contain 145.946 grams of carbon,
24.3243 grams of hydrogen and the rest is oxygen. What is the empirical
formula for the compound?
6. MOLECULAR FORMULA PROBLEMS – molecular formulas are those formulas that
show the actual number of every different atom present and not just the ratio between the atoms
like the empirical formula does; molecular formulas could be reduced but they are not.
Before you can work a molecular formula problem, you must first be given 2 things:
1. the molecular mass of the compound
2. the empirical formula of the compound or the means to find the empirical formula
How to find the molecular formula:
Step 1: Make sure you have the empirical formula. If you don’t, follow the steps for
finding an empirical formula.
Step 2: Find the molecular mass of the empirical formula
Step 3: Divide the given molecular mass by the molecular mass of the empirical formula.
The result here should be a whole number or very, very close to a whole number.
Step 4: Use this whole number result to multiply all of the subscripts of the empirical
formula by and write the molecular formula.
Example 35: If the empirical formula for a carbohydrate is CH2O and its molecular mass is
known to be 240 g/mol, what is the molecular formula for the carbohydrate?
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Example 36: If the empirical formula for a hydrocarbon is known to contain 92.3% carbon
and the molecular mass is known to be 78 g/mol, what is the molecular formula for the
compound?
Example 37: A certain hydrocarbon was found to be 80% carbon and 20% hydrogen. The
molecular weight was determined to be 30. What is the molecular formula?
7. HYDRATE PROBLEMS – hydrates are compounds that trap water in their crystal structures
Before you can work a hydrate problem, you must be sure that you know 2 things:
1. the mass of the anhydrous (dry; water already driven off; dehydrated) compound
2. the mass of the water that was trapped and driven off with heat
How to solve hydrate problems:
Step 1: Divide the mass (or the percentage) of the dry compound by the formula mass of
the ionic compound only (not the formula mass of the entire hydrate) and leave
your answer in at least 4 significant figures.
Step 2: Divide the mass (or the percentage) of the water by the molecular mass of water
that is always 18 g/mol and leave your answer in at least 4 significant figures.
Step 3: Compare the values you obtained in #1 and #2 by selecting the smaller of the two
and dividing both of them by it. Your result here should be very, very close to a
whole number, and this is the number of moles of water that the compound traps
Step 4: Write the formula of the ionic compound followed by a dot and then write the
number of water molecules you just calculated as the correct formula of the
hydrate.
Example 38: A chemist heats a 300 gram sample of hydrated iron(III) nitrate until all of the
water of hydration has been driven off. The anhydrous (dry) compound is found to weigh
179.7 grams. What is the correct name of the hydrate?
Example 39: A hydrate of iron(II) sulfite is known to contain 44.26% water. What is the
correct formula of the hydrate?
Example 40: When 500 grams of hydrated copper(II) sulfate were heated strongly in a
crucible, 180.35 grams of water were released and the anhydrous copper(II) sulfate remained
in the crucible. What is the correct formula for the hydrate?
5F - MOLECULAR FORMULA AND HYDRATE WORKSHEET
1. A chemist has a sample of hydrated Li2SiF6 and it weighs 0.4813 grams.
He heats it strongly to drive off the water of hydration, and after heating,
he finds that the anhydrous compound has a mass of 0.391 grams. Find
the formula of the hydrate.
2. A chemist has a sample of hydrated magnesium sulfite which weighs
1.50 grams. After heating it strongly, he finds that 0.763 grams of water
have been driven off. Find the correct formula of the hydrate and name it.
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3. A chemist finds by experimentation that a certain compound contains
76.9% dry calcium sulfite and 23.1% water of hydration (by mass) in its
crystal. What is the formula of the hydrate?
4. A chemist heats a sample of hydrated lithium nitrate which weighs 170
grams and finds, after heating, that it weighs 95.3 grams. Find the correct
formula of the hydrate and name it.
5. A chemist heats a sample of barium bromide and finds that it gives off
10.8 grams of water. The compound which is left in the crucible after
heating weighs 89.2 grams. What is the formula of the hydrate?
6. A 2.9251-pound sample of hydrated cadmium sulfate is heated and it
gives off 1.1101 pounds of water. What is the name of the hydrate?
7. 5.5771 pounds of anhydrous Li2PtCl6 are left after heating the lithium
hexochloroplatinate hydrate strongly for 1.0 hour. 1.4273 pounds of water
are given off during the heating process. Write the correct formula for the
hydrate.
8. What is the molecular formula of dichloroacetic acid if the empirical
formula is CHCl2COOH and the molecular mass is 129 g/mole?
9. What is the molecular formula of cyanogen chloride if the empirical
formula is CNCl and the molecular mass is known to be 184.5 g/mol?
10. The empirical formula for a compound contains one atom of thallium,
two atoms of carbon, three atoms of hydrogen and two atoms of oxygen.
The molecular mass of the compound is known to be somewhere between
500 and 600 g/mol. What is the molecular formula of the compound?
11. A binary hydrocarbon is known to contain 92.3% carbon. If the
molecular mass of this compound is known to be between 75 and 80
g/mol, what is the molecular formula?
12. Find the molecular formula of a compound containing three elements if
it is known that 26.7% of the compound is phosphorus, 12.1% is nitrogen
and the rest is chlorine. The molecular mass is 695 g/mol.
13. The ratio of carbon atoms to hydrogen atoms to oxygen atoms in a
molecule of THC (the active ingredient in marijuana) is 11:15:1. The
molecular mass of THC is 326 g/mol. What is the molecular formula for
tetrahydrocannabinol, THC?
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5G - MOLE, EMPIRICAL FORMULA, PERCENTAGE COMPOSITION
1. How much would 6.7 x 1028 molecules of manganese(II) chloride weigh
in grams?
2. How many atoms are there in 100 grams of zinc?
3. How many moles are there in 406 grams of copper(II) nitrate?
4. How much would 100 molecules of water weigh in grams?
5. How many molecules are there in 43 grams of sodium hydroxide?
6. Convert 33.7 grams of iron(III) oxide to moles.
7. How many moles are there in 16.5 grams of potassium cyanide?
8. Convert 11.9 moles of barium chlorate to grams.
9. How many atoms would there be in a pure iron nail weighing 75 grams?
10. Find the empirical formula for a compound that is 33.3% calcium,
26.7% sulfur, and the rest of the compound is oxygen. If the molecular
mass is 120.2 amu, what is the molecular formula?
11. What is the empirical formula of a compound containing 1.21 grams of
aluminum, 1.88 grams of nitrogen and 6.44 grams of oxygen?
12. Find the empirical formula if a compound is analyzed and found to
contain 83.4% mercury, 6.6% sulfur, and 9.96% oxygen. If the molecular
mass is 1443.9 amu, what is the molecular formula?
13. Find the empirical formula of a compound if it is known that it contains
only carbon and hydrogen and the percent hydrogen is 7.7%.
14. Find the percentage composition of:
a) silicon carbonate
b) mercury(II) nitride
c) lead(II) hydroxide
15. A compound was found to contain 287 grams of lead and 44.4 grams
sulfur. Its total molecular mass is 478 amu. Find its molecular formula.
16. What is the molecular formula for a compound whose empirical
formula is CH2O3N if its molecular mass is 304 amu?
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CHEMISTRY
2013-2014
5H - "I HATE THESE KIND OF PERCENTAGE COMPOSITION PROBLEMS"
1. Randall students need money! Gold is currently valued at $251 per Troy
ounce. (31.103 g = 1 Troy ounce) They hope to raise money by
decomposing the compound gold(III) chloride into its component parts
and selling the pure gold at its current market price. How many grams of
gold(III) chloride must they decompose in order to sell the pure gold for
$250?
2. If a burglar broke into the chemical storeroom and stole 1.33 kg of
silver nitrate, decomposed all of it into its components and sold the silver
he obtained for $2.93 per Troy ounce of silver, how much money would he
have?
3. A booster club decided to raise money by selling iron. They scraped
rust (iron(III) oxide) from every old car and garden tool they could find in
Amarillo and were able to get 500 lbs of rust that they could decompose. If
iron is currently valued at $1.09 per pound, how much money did they
raise for the club?
4. The Science Team needs money to go to Wichita Falls to contest. They
have decided to decompose mercury(I) chloride and sell the pure mercury
at its current value of $65 per pound. How many grams of mercury (I)
chloride must they decompose in order to raise enough money to pay their
travel expenses of $400? Would it have been smarter to decompose the
same amount of mercuric chloride? Why or why not?
5. A scientist bragged that he had sold the pure copper he had obtained
by decomposing a copper compound for $40. Copper is currently valued
at $3.50 per pound. If he decomposed 10990 grams of the copper
compound to obtain this much copper, what is the percent of copper in
the compound he decomposed? Is the compound he decomposed more
likely cupric chloride or cuprous chloride and how do you know?
6. The Student Council is trying to raise money. They bought 100 kg of
magnesium chlorate for $1.00 per kilogram and then decomposed the
compound into its components and sold the pure magnesium for $2.95 per
pound. How much money did they NET?
7. Brass is 72% copper. If the value of pure copper is $3.50 per pound,
how much could you earn by selling the pure copper in 1000 pounds of
brass?
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