Eng Chem
Homework #1
Due date: 22/03/2011
Average Atomic Mass Problems
1. What is average atomic mass of Lithium if 7.42% exists as 6Li (6.015 g/mol) and 92.58%
exists as 7Li (7.016 g/mol)?
2. Magnesium has three naturally occuring isotopes. 78.70% of Magnesium atoms exist as
Magnesium-24 (23.9850 g/mol), 10.03% exist as Magnesium-25 (24.9858 g/mol) and 11.17%
exist as Magnesium-26 (25.9826 g/mol). What is the average atomic mass of Magnesium?
3. Neon has two major isotopes, Neon-20 and Neon-22. Out of every 250 neon atoms, 225 will
be Neon-20 (19.992 g/mol), and 25 will be Neon-22 (21.991 g/mol). What is the average
atomic mass of Neon?
4. What is the atomic mass of Hafnium if out of every 200 atoms, 10 have mass 176.00 g/mol,
38 have mass 177.00 g/mol, 54 have mass 178.00 g/mol, 28 have mass 179.00 g/mol, and 70
have mass 180.00 g/mol?
5. In a sample of 200 Chlorine atoms, it is found that 151 are 35Cl (34.969 g/mol), and 49 are
37
Cl (36.966 g/mol). What is the average atomic mass of Chlorine?
6. Without doing any math, are there more Bromine-79 atoms or more Bromine-80 atoms on
earth? (Hint: look at the periodic table.)
Stoichiometry Problems
1a) How many moles of chlorine gas (Cl2) would react with 5 moles of sodium (Na) according
to the following chemical equation? (Balance equation.)
Na + Cl2 --> NaCl
1b) Using the equation (after it is balanced) above, determine the amount of product that can be
produced from 24.7 g Na.
1c) How many molecules of product would be produced from 24.7g Na?
__________________________________________________________________________________
2a) In the reaction 2C8H18 + 25O2 --> 16CO2 + 18 H2O, the ratio of volumes of O2 to CO2
is _________________.
2b) If 27.3g of C8H18 are combusted, what mass of water will be produced?
2c) How many molecules of CO2 will be produced?
2d) How many atoms of H are in 2 mol of C8H18?
2e) What is the percentage, by mass, of the H in 2 mol of C8H18?
Limiting Reagent Worksheet
All of the questions on this worksheet involve the following reaction: When copper (II)
chloride reacts with sodium nitrate, copper (II) nitrate and sodium chloride are formed.
1)
Write the balanced equation for the reaction given above:
2)
If 15 grams of copper (II) chloride react with 20 grams of sodium nitrate, how much
sodium chloride can be formed?
3)
What is the limiting reagent for the reaction in problem #2? ____________
4)
How much of the nonlimiting reagent is left over in this reaction?
5)
If 11.3 grams of sodium chloride are formed in the reaction described in problem #2,
what is the percent yield of this reaction?
Molarity Practice Problems
1)
How many grams of potassium carbonate are needed to make 200 mL of a 2.5 M
solution?
2)
How many liters of 4 M solution can be made using 100 grams of lithium bromide?
3)
What is the concentration of a 450 mL solution that contains 200 grams of iron (II)
chloride?
4)
How many grams of ammonium sulfate are needed to make a 0.25 M solution at a
concentration of 6 M?
5)
What is the concentration of a solution that has a volume of 2.5 L and contains 660
grams of calcium phosphate?
6)
How many grams of copper (II) fluoride are needed to make 6.7 liters of a 1.2 M
solution?
How many liters of 0.88 M solution can be made with 25.5 grams of lithium fluoride?
7)
8)
What is the concentration of a solution that with a volume of 660 that contains 33.4
grams of aluminum acetate?
9)
How many liters of 0.75 M solution can be made using 75 grams of lead (II) oxide?
10)
How many grams of manganese (IV) oxide are needed to make a 5.6 liters of a 2.1 M
solution?
11)
What is the concentration of a solution with a volume of 9 mL that contains 2 grams of
iron (III) hydroxide?
12)
How many liters of 3.4 M solution can be made using 78 grams of isopropanol
(C3H8O)?
13) What is the concentration of a solution with a volume of 3.3 mL that contains 12 grams
of ammonium sulfite?
Dilutions Worksheet
1)
If I have 340 mL of a 0.5 M NaBr solution, what will the concentration be if I add 560
mL more water to it?
2)
If I dilute 250 mL of 0.10 M lithium acetate solution to a volume of 750 mL, what will
the concentration of this solution be?
3)
If I leave 750 mL of 0.50 M sodium chloride solution uncovered on a windowsill and
150 mL of the solvent evaporates, what will the new concentration of the sodium
chloride solution be?
4)
To what volume would I need to add water to the evaporated solution in problem 3 to
get a solution with a concentration of 0.25 M?