1. STOICHIOMETRY
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5.6 – To determine, using stoichiometric calculations, the quantity of a substance involved in a chemical reaction.
5.8 – To solve numerical problems related to stoichiometry.

2. Recall:
A mole is the amount of substance that contains 6.02 x 1023 particles of that substance
n (moles) = mass of substance (g)
molar mass of substance (g/mol)

3. Stoichiometry: Stoichiometry:
uses ratios of a chemical reaction to determine the amounts of elements/ compounds (reactants & products) involved in a chemical change

4. Ex. 2Mg + O2  2MgO 2 : 1 : 2 Mole Ratio –
Magnesium metal reacts with oxygen to produce magnesium oxide.
2Mg + O2  2MgO
2 : 1 : 2
Mole Ratio –
describes the ratio in which the substances combine
Ex. for every 2 Mg, 1 O2 needed
used to predict what will happen
Ex. for every 2 Mg & 1 O2, 2 MgO will be produced

5. Ex #1 First Second 2Mg + O2  2MgO 2 : 1 : 2
4 moles of magnesium metal reacts with oxygen to produce magnesium oxide. How many moles of MgO are produced?
2Mg + O2  2MgO
First
Mole Ratio
2 : 1 : 2
Second
from mole ratio
Set up ratio
2 moles Mg
2 moles MgO =
4 moles Mg
? moles MgO
from question

6. Ex #1 cont... Third 2 moles Mg 2 moles MgO = 4 moles Mg ? moles MgO
Solve by cross multiplying
2 moles Mg
2 moles MgO =
4 moles Mg
? moles MgO
4 x 2 ÷ 2
4 moles MgO are produced

7. Ex #2 First Second 4Fe + 3O2  2Fe2O3 4 : 3 : 2
Iron metal reacts with oxygen to produce 3 moles of iron (III) oxide. How many moles of oxygen were needed in the reaction?
4Fe + 3O2  2Fe2O3
First
Mole Ratio
4 : 3 : 2
Second
from mole ratio
Set up ratio
3 moles O2
2 moles Fe2O3 =
? moles O2
3 moles Fe2O3
from question

8. Ex #2 cont... Third 3 moles O2 2 moles Fe2O3 = ? moles O2
Solve by cross multiplying
3 moles O2
2 moles Fe2O3 =
? moles O2
3 moles Fe2O3
3 x 3 ÷ 2
4.5 moles O2 are needed

9. Ex #3
How many moles of nitrogen gas & hydrogen gas are required to make 7.5 mol of ammonia?
N2 + 3H2  2NH3
1 : 3 : 2
Solve for moles of N2:
11.25 moles H2 are required
1 mole N2
2 moles NH3 =
? moles N2
7.5 moles NH3
Solve for moles of H2:
3.75 moles N2 are required
3 mole H2
2 moles NH3 =
? moles H2
7.5 moles NH3

10. Ex. Ex. We can now add mass into problems involving stoichiometry.
Recall,
n (moles) = mass of substance (g) molar mass of substance (g/mol)
Ex.
How many moles are in 12.5g of Ca?
n (moles) = 12.5g
40g/mol
mol of Ca
Ex.
How many moles are in 2.4g of NO2?
n (moles) = 2.4g
46g/mol
0.052 mol of NO2

11. Ex #4 First Second: 2Li + F2  2LiF 2 : 1 : 2
How many moles of Li are needed to react with fluorine gas to make 435g of lithium fluoride?
2Li + F2  2LiF
2 : 1 : 2
First
Solve for moles of LiF:
Second:
n (moles) = 435g g/mol
Solve for moles of Li:
16.73 mol of LiF
2 moles Li
2 moles LiF =
? moles Li
16.73 mol LiF
16.73 moles Li are needed

12. Ex #5 First Second 2NaClO3  2NaCl + 3O2 2 : 2 : 3
How many grams of NaClO3 must be decomposed to produce sodium chloride & 1.65 moles of oxygen?
Ex #5
2NaClO3  2NaCl + 3O2
2 : 2 : 3
First
Solve for moles of NaClO3:
2 mol NaClO3
3 mol O2 =
? mol NaClO3
1.65 mol O2
1.1 mol of NaClO3
Second
Solve for mass of NaClO3:
116.6g of NaClO3
1.1 mol = ? g g/mol

13. Ex #6 First Third Second Mg + 2HCl  MgCl2 + H2 1 : 2 : 1 : 1
3g of magnesium react with how many grams of hydrochloric acid to produce hydrogen gas & magnesium chloride?
Ex #6
Mg + 2HCl  MgCl2 + H2
1 : 2 : 1 : 1
First
Solve for moles of Mg:
Third
n = g g/mol
Solve for mass of HCl:
0.25mol = ? g g/mol
0.125 mol of Mg
Second
9g of HCl
Solve for moles of HCl:
1 mol Mg
2 mol HCl =
0.25 mol of HCl
0.125 mol Mg
? mol HCl

14. Key Points: Always find moles first!!!
Use mole ratio to find the moles of each reactant & product
Remember, the equation must be balanced to give you the mole ratio
n (moles) = mass of substance (g)
molar mass of substance (g/mol)

15. Practice:
Worksheet #