1. Stoichiometric Calculations
12.2
Stoichiometric Calculations
A car’s air bags are powered by the rapid reaction of a small mass of sodium azide (Na3N) into a large volume of N2 gas.
(all in less than a second)
You will use a balanced
chemical equation
to calculate the amount
of reactant needed or
product formed.
Print 1-8,10-13

2. 12.2
Mol to Mol Ratios
In chemical calculations, mol ratios convert moles of one substance to moles of another substance.

3. 2 H2 + O2 → 2 H2O 2 mol H2 for every 1 mol O2 Mol to Mol Ratios
12.2
Mol to Mol Ratios
mol ratio: a conversion factor from the coefficients of a balanced equation.
2 H O2 → 2 H2O
2 mol H2 for every 1 mol O2

4. 2 H2 + O2 → 2 H2O OR 2 mol H2 for every 2 mol H2O Mol to Mol Ratios
12.2
Mol to Mol Ratios
mol ratio: a conversion factor from the coefficients of a balanced equation.
2 H O2 → 2 H2O OR
2 mol H2 for every 2 mol H2O

5. 2 H2 + O2 → 2 H2O OR 1 mol O2 for every 2 mol H2O Mol to Mol Ratios
12.2
Mol to Mol Ratios
mol ratio: a conversion factor from the coefficients of a balanced equation.
2 H O2 → 2 H2O OR
1 mol O2 for every 2 mol H2O

6. 12.2
2 H O2 → 2 H2O
There are 6 mol to mol ratios that can be written from this balanced equation:
2 mol H2 1 mol O2 2 mol H2O
1 mol O2 2 mol H2 2 mol H2
2 mol H2O 2 mol H2O 1 mol O2
To determine the number of moles in a sample of a compound, first measure the mass of the sample. Then use the molar mass to calculate the number of moles in that mass. 6

7. We must use MOLES to calculate, but…
12.2
We must use MOLES to calculate, but…
We don’t measure moles in the lab, we measure grams.
To get moles in a sample:
measure the mass (g), then
convert grams to moles
using Molar Mass (g/mol)
To determine the number of moles in a sample of a compound, first measure the mass of the sample. Then use the molar mass to calculate the number of moles in that mass.

8. Stoichiometric Calculations
In a typical stoichiometric problem,
1) Convert grams to moles
(if necessary)
2) Use the mol ratio to convert
moles of given to moles of wanted
3) Convert moles to grams
(or any unit)

9. Mg + 2 HCl → MgCl2 + H2 1 mol H2 1 mol Mg 4.0 mol Mg x = 4.0 mol H2
12.2
Moles to Moles (Calculation #1)
How many moles of hydrogen gas (H2) are produced from 4.0 mol Mg?
Mg HCl → MgCl2 + H2
1 mol H2
1 mol Mg
4.0 mol Mg x =
4.0 mol H2
mol ratio 9

10. Mg + 2 HCl → MgCl2 + H2 1 mol H2 2 mol HCl 4.0 mol HCl x = 2.0 mol H2
12.2
Moles to Moles (Calculation #1)
How many moles of hydrogen gas (H2) are produced from 4.0 mol HCl?
Mg HCl → MgCl2 + H2
1 mol H2
2 mol HCl
4.0 mol HCl x =
2.0 mol H2
mol ratio 10

11. Mg + 2 HCl → MgCl2 + H2 mol ratio molar mass
12.2
Moles to Grams (Calculation #2)
How many grams of H2 are produced from 4.0 mol HCl?
Mg HCl → MgCl2 + H2
1 mol H2
2 mol HCl
2.02 g H2
1 mol H2
4.0 mol HCl x x =
4.04 g H2
mol ratio
molar mass 11

12. Mg + 2 HCl → MgCl2 + H2 molar mass mol ratio
12.2
Grams to Moles (Calculation #3)
How many mol of H2 are produced from
48 g Mg?
Mg HCl → MgCl2 + H2
1 mol Mg
24.31 g Mg
1 mol H2
1 mol Mg
48 g Mg x x =
1.97 mol H2
molar mass
mol ratio 12

13. Mg + 2 HCl → MgCl2 + H2 mol ratio molar mass molar mass
12.2
Grams to Grams (Calculation #4)
How many grams of H2 are produced from
18 g HCl?
Mg HCl → MgCl2 + H2
1 mol HCl
36.46 g HCl
1 mol H2
2 mol HCl
2.02 g H2
1 mol H2
18 g HCl x x x =
0.499 g H2
mol ratio
molar mass
molar mass 13

14. C2H5OH(l) + 3 O2(g)  2 CO2(g) + 3 H2O(g)
Quick Quiz!
1. How many moles of water are produced from burning 4.00 mol of ethanol (C2H5OH)?
C2H5OH(l) + 3 O2(g)  2 CO2(g) + 3 H2O(g)
3.00 mol H2O
4.00 mol H2O
12.0 mol H2O
1.33 mol H2O

15. C2H5OH(l) + 3 O2(g)  2 CO2(g) + 3 H2O(g)
Quick Quiz.
2. How many grams of oxygen gas (O2) are needed to react with 2.00 mol of ethanol (C2H5OH)?
C2H5OH(l) + 3 O2(g)  2 CO2(g) + 3 H2O(g)
6.00 g O2
21.1 g O2
96.0 g O2
192 g O2