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Mass Relationships in
Chemical Reactions
Chapter 3
Natural lithium is:
7.42% 6Li (6.015 amu)
92.58% 7Li (7.016 amu)
Average atomic mass of lithium:
100
= _______ amu
3.1
The mole (mol) is the amount of a substance that
contains as many elementary entities as there
are atoms in exactly ______ grams of 12C
1 mol = NA =____________________
___________________ number (NA)
3.2
eggs
Molar mass is the mass of 1 mole of shoes in grams
marbles
atoms
1 mole 12C atoms = ____________ atoms = ______ g
1 12C atom = 12.00 amu
1 mole 12C atoms = ___________ g 12C
1 mole lithium atoms = ___________ g of Li
For any element
___________ (amu) = __________ (grams)
3.2
One Mole of:
S
C
Hg
Cu
Fe
3.2
_____ g = ________________ amu
1 amu = ______________ g
M = molar mass in g/mol
NA = Avogadro’s number
3.2
Do You Understand Molar Mass?
How many atoms are in 0.551 g of potassium (K) ?
3.2
Molecular mass (or molecular weight) is the sum of
the atomic masses (in amu) in a molecule.
SO2
1S
2O
amu
amu
SO2
amu
For any molecule
________ mass (amu) = ______ (grams)
1 molecule SO2 = _________ amu
1 mole SO2 = ________ g SO2
3.3
Do You Understand Molecular Mass?
How many H atoms are in 72.5 g of C3H8O ?
3.3
Percent composition of an element in a compound =
n x molar mass of element
x 100%
molar mass of compound
n is the number of moles of the element in 1 mole
of the compound
C2H6O
3.5
Types of Formulas
• ____________ Formula
The formula of a compound that
expresses the _________________ ratio of
the atoms present.
Ionic formula are always ____________
formula
• _______ Formula
The formula that states the _____
number of each kind of atom found in
_________ of the compound.
To obtain an Empirical Formula
1. Determine the _______________ of each
element present, if necessary.
2. Calculate the number of _________ of
each element.
3. Divide each by the smallest number of
moles to obtain the_______________.
4. If _____numbers are not obtained* in step
3), multiply through by the _____ number
that will give all whole numbers
* Be
careful! Do not round off numbers prematurely
A sample of a brown gas, a major air pollutant,
is found to contain 2.34 g N and 5.34g O.
Determine a formula for this substance.
require mole ratios so convert grams to moles
Calculation of the Molecular Formula
A compound has an empirical formula of
NO2. The colourless liquid, used in rocket
engines has a molar mass of 92.0 g/mole.
What is the molecular formula of this
substance?
Empirical Formula from % Composition
A substance has the following composition by
mass: 60.80 % Na ; 28.60 % B ; 10.60 % H
What is the empirical formula of the substance?
Consider a sample size of _____ grams
This will contain: _____ grams of Na,
______ grams of B, and _____ grams H
Determine the number of ______ of each
Determine the ________ number ratio
Combust 11.5 g ethanol
Collect 22.0 g CO2 and 13.5 g H2O
g CO2
mol CO2
mol C
gC
6.0 g C = 0.5 mol C
g H2O
mol H2O
mol H
gH
1.5 g H = 1.5 mol H
g of O = g of sample – (g of C + g of H)
4.0 g O = 0.25 mol O
Empirical formula C0.5H1.5O0.25
Divide by smallest subscript (0.25)
Empirical formula C2H6O
3.6
Mass Changes in Chemical Reactions
1. Write balanced chemical equation
2. Convert quantities of known substances into moles
3. Use coefficients in balanced equation to calculate the
number of moles of the sought quantity
4. Convert moles of sought quantity into desired units
3.8
Other units
• Molarity
– Moles solute / L solution
• Gases
– 22.4 L = 1 mole of ANY GAS at STP
Methanol burns in air according to the equation
2CH3OH + 3O2
2CO2 + 4H2O
If 209 g of methanol are used up in the combustion,
what mass of water is produced?
3.8
Limiting Reagents
6 red
green
leftused
overup
3.9
Method 1
•
•
•
•
Pick A Product
?????
The ____ answer will be the correct answer
The _____that gives the _____answer will
be the _______ reactant
Limiting Reactant: Method 1
• 10.0g of aluminum reacts with 35.0 grams of chlorine gas
to produce aluminum chloride. Which reactant is
limiting, which is in excess, and how much product is
produced?
2 Al + 3 Cl2  2 AlCl3
• Start with Al:
• Now Cl2:
Method 2
• Convert one of the reactants to the _____
• See if there is enough _______ to use up the
other ______
• If there is less than the ______amount, it is
the ______ reactant
• Then, you can find the desired species
Do You Understand Limiting Reagents?
In one process, 124 g of Al are reacted with 601 g of Fe2O3
2Al + Fe2O3
Al2O3 + 2Fe
Calculate the mass of Al2O3 formed.
3.9
Use limiting reagent (Al) to calculate amount of product that
can be formed.
g Al
mol Al
mol Al2O3
g Al2O3
3.9
Finding Excess Practice
• 10.0g of aluminum reacts with 35.0 grams of
chlorine gas
2 Al + 3 Cl2  2 AlCl3
• We found that _______ is the limiting reactant, and
_______ of aluminum chloride are produced.
Given amount
of excess
reactant
Amount of
excess
reactant
actually
used
Note that we started with
the limiting reactant! Once
you determine the LR, you
should only start with it!
___________ is the amount of product that would
result if all the limiting reagent reacted.
_________ is the amount of product actually obtained
from a reaction.
% Yield =
_________Yield
x 100
_________Yield
3.10