THE MOLE - Chemistry

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CHAPTER 10
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State with 100% accuracy Avogadro's number
as 6.02x1023 particles
Apply Avogadro's number to convert
between particles of an element and grams of
an element with increasing accuracy (mole
conversions)
Is it:
a. A Mexican sauce.
b. A skin imperfection.
c. A small, burrowing mammal.
d. A unit for counting in chemistry.
All of the above!
Chemists use the mole to count
the number of atoms, molecules,
ions and compounds they work
with.
From the German word “mol” which
is short for molekulargewicht
(molecular weight) and the French
word moléculaire (molecule)
Chemists need a more convenient
method for accurately counting the
number of atoms, ions, molecules
and compounds in their work.
Century
Week
Gross
Ream
Dozen
Acre
=
=
=
=
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1 mole = 6.02 x 1023 particles OR
602,213,670,000,000,000,000,000
A particle may be an atom, ion,
molecule, compound, etc.
Ex: Na, Na+, NaCl, Cl2
6.02 x 1023 is called Avogadro’s number.
Amadeo Avogadro- Italian physicist
(1776-1856)
In 1811 determined the number of
atoms of 1 mole of a gas to be
6.02 x 1023 atoms.
1f one mole of pennies were divided up
and given to every person on the earth,
each person would receive 1.5 x 1016
pennies.
At a spending rate of 1 million dollars per
day, it would take each person 4
thousand years to spend a mole of
pennies.
1 mole of basketballs would fill up a ball
bag the size of the earth.
1 mole of marshmallows would cover the
US to a depth of 600 miles.
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If you have 2 dozen roses, how many roses do
you have?
If you have 3.6 dozen chocolates how many
chocolates do you have?
If you have 82 roses, how many dozen roses
do you have?
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“Representative Particles” are either atoms,
formula units or molecules
Atoms  Elements
Formula Units (F.U.’s)  Ionic Compounds (is
there a metal present?)
Molecules  Covalent Compounds
(nonmetals only)
 Your conversion will always be
either 1 mol/6.02 x 1023 or vice versa
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How many sodium atoms are in a mole?
If you have 2 moles of sodium atoms, how
many sodium atoms do you have?
If you have 3.5 moles of sodium atoms, how
many sodium atoms do you have?
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If you have 4.9 x 1023 chlorine atoms, how
many moles of chlorine atoms do you have?
If you have 22.7 x 1027 chlorine atoms, how
many moles of chlorine atoms do you have?
If you have 5.5 x 1021 H2O formula units, how
many moles of H2O formula units do you
have?
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BIG IDEA: All chemists who create or
manufacture compounds/chemicals need to
analyze the stuff/compounds they make to
be sure it is what they say it is.
 Examples: pharmaceuticals, paints, food additives
 % composition data is determined using a small
sample of the solid the chemist has synthesized.
 The chemist than takes the % composition data
and works BACKWARD to determine the
molecular formula of the compound.
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Example:You have a compound which was
analyzed and found to have the following
composition by mass:
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27.37% Na
1.20% H
14.30 % C
57.14% O
Working backward a chemist can determine
the formula of the compound to be NaHCO3
or baking soda.
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Example: You have 2 different ores which
contain gold. You only have the time, money
and equipment to mine for one of the ores.
You need to decide which ore contains the
most pure gold.
 350 lbs. of calaverite containing 40% by mass Au.
 480 lbs. of petzite containing 25% by mass Au.
 Which of the 2 ores do you mine and process?
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Example: A food processing plant
manufactures different sweeteners- glucose
(C6H12O6 ) and sucrose (C12H22O11).
Some numbskull in the lab got the two
products mixed up. The % composition
analysis shows the following:
% carbon = 40.0%
% hydrogen = 6.7%
% oxygen = 53.3 %
Is the product sucrose or glucose?
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A formula giving the proportions of the
elements present in a compound but not the
actual numbers or arrangement of atoms.
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Please Write this down and use often!!!
Steps to finding an empirical formula
 Change all percents to grams
 Convert all grams to moles
 Divide by the smallest number of moles
 If all your numbers are whole, you are done
▪ If a number ends in .5, multiply all answers by 2
▪ If a number ends in .33 or .66, multiply all answers by 3
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To find the molecular formula from the
empirical formula, they must give you the
molar mass or the molecular formula.
Divide the molar mass of the molecular
formula by the molar mass of the empirical
formula.
Take that number and multiply by everything
within the molecular formula.
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BIG IDEA: Hydrates are solid ionic
compounds in which molecules of water are
trapped.
Dessicants are chemical compounds which
absorb water to form a hydrate.
 Examples: SiO2, CaCO3 – these are dessicants
added to products such as shoes and electronics
before shipment overseas.
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Formulas of hydrates contain a chemical
formula followed by a dot and the # of H2O
molecules for the hydrate.
Example:
Na2CO3•10H2O
called sodium carbonate
decahydrate
NH4OH•H2O
called ammonium
hydroxide monohydrate
CaCl2•2H2O
called calcium chloride
dihydrate
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1 – Mono
3 – Tri
5 – Penta
7 – Hepta
9 – Nona
2 – Di
4 – Tetra
6 – Hexa
8 – Octa
10 - Deca
Practice
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CuSO4•5H2O
MgSO4•6H2O
MgSO4 •7H2O
Calcium hydroxide octahydrate
barium hydroxide octahydrate
barium chloride dihydrate
FePO4•4H2O
CaCl2 •2H2O
Cd(NO3)2 • 6H2O
Calcium chloride trihydrate
lithium chloride tetrahydrate
sodium sulfate pentahydrate
Weigh the hydrate (in problems, this is given to you)
Drive off the water and re-weigh what’s left. (Once
you’ve driven off the water, the compound is now
called an “anhydrate” or “anhydrous”)
 Calculate the weight of the compound and the
weight of the water driven off.
 Convert both to moles
 Come up with a ratio of water to compound
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A 15.67 g sample of a hydrate of magnesium
carbonate was heated, without decomposing
the carbonate, to drive off the water. The
mass was reduced to 7.58 g. What is the
formula of the hydrate?
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A hydrate of Na2CO3 has a mass of 4.31 g
before heating. After heating, the mass of the
anhydrous compound is found to be 3.22 g.
Determine the formula of the hydrate and
then write out the name of the hydrate.
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A 5.00 g sample of hydrated barium chloride,
BaCl2 · nH2O, is heated to drive off the water.
After heating, 4.26 g of anhydrous barium
chloride, BaCl2, remains. What is the value of
n in the hydrate's formula?
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BIG IDEA: Stoichiometry is the study of
quantitative relationships between the
amounts of reactants used and the amounts
of products formed by a chemical reaction.
The amount of each reactant present at the
start of a chemical reaction determines how
much product can form.
Based on the law of conservation of mass!
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Using stoichiometry allows chemists and
engineers to calculate how much reactant or
starting materials they must buy and how
much product they will produce BEFORE the
reaction even takes place.
Chemicals and materials are expensive!
Knowing how much you will need and how
much you will produce is more efficient and
saves money!
STOICHIOMETRY (or stoich for short) derives
from the Greek word “stoikheion”, meaning
“element” + “-metry”, meaning to measure.
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Stoichiometry requires the use of balanced
chemical equations and molar masses of
chemical compounds!
A mole ratio is the ratio between the
numbers of moles of any two of the
substances in a balanced chemical equation.
 The mole ratio comes from the coefficients in
the balanced chemical equation.
 The mole ratio relates the masses of
reactants to products using the molar
masses.
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Example:
4Fe (s) + 3O2 (g) ―> 2Fe2O3 (s)
4 moles of Fe and 3 moles of O2 are
required to produce 2 moles of 2Fe2O3.
 In other words- moles of Fe to O2 to Fe2O3
can be represented using the mole ratios
or fractions4Fe
3O2
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3O2
2Fe2O3
• Reactions proceed until one of the reactants
is used up, leaving the other reactant(s) in
excess.
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The limiting reactant limits the extent of the
reaction and, thereby, determines the
amount of product formed.
The excess reactants are all the leftover
unused reactants.
Pick one of the products (if there’s only one, use that one) –
perform a gram-to-gram calculation from both reactants
over to that product (two separate problems)
 Whichever answer is smaller, that reactant that you started
with is the limiting reagent (the other is the excess reagent)
 To find the amount of excess reagent used, start with grams
of the LR and do a gram-to-gram from the limiting to the
excess reagent. If they want how much of the excess
reagent is left over, you subtract the answer from the
original amount.
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LR and ER can also be called “limiting reactant” and “excess
reactant”
Silicon nitride, Si3N4, is used in the manufacturing of hightemperature thermal insulation for heat engines and
turbines. It is produced by the following chemical reaction:
__Si + __N2 ―> __ Si3N4
1. How many grams of Si3N4 can you produce if you start
with 100 g of silicon, Si?
2. How many grams of Si3N4 can you produce if you start
with 100 g of nitrogen, N2?
3. If you have 100 g of Si and 100g of N2, how much Si3N4
can you actually produce? _________________
What will the limiting reactant be? ________________
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% yield- a measure of the efficiency of a
chemical reaction.
Theoretical yield- the use of stoichiometry
calculations to predict the amount of product
you can obtain from the reactant(s).
Actual yield- the amount of product produced
when the chemical reaction is actually carried
out in the lab.
% Yield =
Actual Yield
X 100
Theoretical Yield
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Solid silver chromate (Ag2CrO4) forms when
potassium chromate (K2CrO4) is added to a solution
containing 0.500 g of silver nitrate (AgNO3).
Calculate the amount of Ag2CrO4 produced.
__K2CrO4 + __AgNO3 ―> __Ag2CrO4 + __KNO3
Calculate the % yield for the reaction if you actually
produce 0.455 g Ag2CrO4 in the lab.
 If you start with 0.500 g of AgNO3 and 0.500 g of
K2CrO4 how much Ag2CrO4 can you actually
produce? Does this change your % yield you
calculated above?
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