January 26, 2014
Ch 12: Stoichiometry
Relationships in chemical reactions
Stoichiometry:
-The study of the quantitative relationships between the
amount of reactants used and the products formed in a
chemical reaction
-based on Law of Conservation of Mass
-total mass of reactants=total mass of products.
Mole Ratios
Note: Any set of 2 coefficients can be set up
as a conversion factor!
4Al(s)+3O2(g)
2Al2O3(s)
If we start with 4 mole Al....we will get 2 mole Al2O3
Therefore: 4 mol Al=2 mol Al2O3
Reaction Stoichiometry Problems
Type 1: Given and unknown quantities are in moles:
(Mole-Mole)
Type 2: Given is an amount in moles and the unknown
is mass expressed in grams. (Moles-Grams)
Type 3: Given is a mass in grams and the unknown is
in moles. (Grams-Moles)
Type 4: Given is in grams and unknown is a mass in
grams. (Mass-Mass)
Type 5: Given is liters and unknown is liters.
(Volume-Volume)
Flip Chart
Mols A
Mols B
Mols B
closed
Mols A
1 mol A
Grams A
___ g A
Mols B
Mols A
Liters A
1 mol A
22.4 L A
___ g B
1 mol B
Grams B
22.4 L B
Liters B
1 mol B
open
January 26, 2014
Section 12.2 Chemical Calculations
In chemical calculations, mole ratios are used to
convert between moles of reaction and moles or
product using mole ratios.
Writing and Using Mole Ratios
N2(g) + 3H2(g)
2NH3(g)
1. Mole-Mole calculation #1
How many moles of ammonia are
produced when 0.60 mol of nitrogen reacts
with hydrogen?
N2(g) + 3H2(g)
2NH3(g)
Mole ratios:
1 mol N
3 mol H
2
2
2 mol NH
1 mol N
2
3
3 mol H
2 mol NH
2
3
1. Mole-Mole calculation #2
In a spacecraft, the carbon dioxide exhaled by
astronauts can be removed by a reaction with lithium
hydroxide, LiOH, according to the following chemical
reaction.
CO2(g) + 2LiOH(s)
2. Mole-Mass Problem #1
What mass of carbon dioxide, in grams, is needed to
react with 3.00 mol of water in the photosynthetic
reaction given?
6CO2(g) + 6H2O(l)
C6H12O6(s) + 6O2(g)
Li2CO3(s) + H2O(l)
How many moles of LiOH are required to react with 20
moles of carbon dioxide, the average amount exhaled by
a person each day?
3.00 mol H2O x 6 mol CO2 x 44.0 g CO2= 132 g CO2
2. Mole-Mass Problem #2
2. Mole-Mass Problem #3
What is the mass of SiC formed from 2
mol of carbon?
SiO2+ 3C
SiC + 2CO
6 mol H2O
1 mol CO2
How many grams of nitrogen can be produced
from the decomposition of 0.5 mol NaN3?
2NaN3
2Na + 3N2
January 26, 2014
3. Mass-Mole Problem #1
2NaCl + 2H2O
2NaOH + Cl2 + H2
250 g NaCl will produce how many moles of chlorine
gas?
4. Mass-Mass Problem #1
Stannous fluoride is used in some toothpastes. It is
made by the reaction of tin with hydrogen fluoride
according to the following equation.
Sn(s) + 2HF(g)
SnF2+ H2(g)
How many grams of SnF2 are produced from 30.0 grams
of HF with tin?
4. Mass-Mass Problem #2
Milk of magnesia, a suspension of Mg(OH)2 in water
reacts with stomach acid, HCl in a neutralization reaction.
What is the balanced equation?
What mass in grams of MgCl2 will be produced if 3.00 g of
Mg(OH)2 reacts?
Section 12.3 Limiting Reagent and Percent Yield
Limiting Reagent: the reagent that determines the amount
of product that can be formed by a reaction.
Excess Reagent: The reactant that is not completely used
up in a reaction
5. Mass-Volume Problem #1
How many liters of hydrogen gas are required to react with
oxygen gas to form 50.0 g of water?
January 26, 2014
Finding the Limiting Reagent
Steps:
1. Turn mass (g) to moles. (If needed)
2. Write mole ratio of reactants in the problem.
3. Compare mole ratio from equation to mole ratio from
problem and determine which in excess and which is
limited.
Finding the Limiting Reagent Practice
Zinc citrate, Zn3(C6H5O7)2 is an ingredient in toothpaste.
It is synthesized by the reaction of zinc carbonate with
citric acid.
3ZnCO3(s) + 2H3C6H5O7(aq)
Zn3(C6H5O7)2(aq) + 3H2O(l) + 3CO2(g)
Questions:
1. If there is 1 mol of zinc carbonate and 1 mol of citric acid
which one is the limiting reactant?
Finding the Limiting Reagent
Silicon dioxide (quartz) is usually quite un-reactive but
reacts readily with hydrogen fluoride according to the
following equation:
SiO2+ 4HF
SiF4 + 2H2O
If 2.0 mol of HF are exposed to 4.5 mol of SiO2, which is
the limiting reagent?
Finding the Limiting Reagent continued:
15.0 g of Copper and 15.0 g of HCl react to form CuCl2
and hydrogen gas.
Cu(s) + 2HCl
CuCl (aq) + H (g)
2
2
Questions:
1. Which reactant is limiting?
2. How many grams of H2 is produced?
3. How much excess Cu will be left over?
2. If there is 6 mol of zinc carbonate and 10 mol of citric acid
which reactant is in excess?
3. How many moles of zinc citrate is produced under the
conditions of number 2.
Percent Yield Calculations
Theoretical Yield
The maximum amount of product that can be produced
by a given amount of reactant.
Note: A chemical reaction rarely produces this amount.
Actual Yield
The quantity that is recovered in the lab.
Percent Yield
% yield: the ratio of the actual yield to the
theoretical yield expressed as a percent.
January 26, 2014
Steps for finding percent yield:
1. Balance the chemical equation
2. Find the limiting reagent
3. Find the theoretical yield (Stoichiometric Conversion!)
4. Find the actual yield (Lab)
5. Find the percentage yield
Example
Methanol can be produced through the reaction of CO and
We need to calculate the theoretical yield from the balanced
equation.
75.0 g CO x 1 mol CO x 1 mol CH3OH x 32.0 g CH3OH
28.0 g CO
1 mol CO
1 mol CH3OH
= 85.7 g
H2 in the presence of a catalyst.
CO(g) + 2H (g)
2
CH OH(l)
3
If 75.0 g of CO reacts to produce CH3OH, what is the
percent yield of CH3OH?
Actual yield= 68.4 g
Percent Yield Example
Quicklime, CaO, can be prepared by roasting
limestone, CaCO3, according to the following reaction.
CaCO3(s)
Δ
CaO(s) + CO2(g)
When 2.00 x 103 g CaCO3 are heated, the actual yield of
CaO is 1.05 x 103 g. What is the percentage yield? (93.8%)
% yield= 68.4 g/ 85.7 g x 100%= 79.8%