1. Chapter 5 Chemical Quantities and Reactions
5.7
Mass Calculations for Reactions

2. Mass Calculations in Equations

3. Moles to Grams Suppose we want to determine the mass (g) of NH3 that
can be produced from 32 grams of N2.
N2(g) + 3H2(g) NH3(g)
Step 1 Use molar mass to convert grams of given to moles.
1 mole of N2 = g of N2
1 mole N and g N2
28.0 g N mole N2
32 g N2 x 1 mole N2 = mole of N2
28.0 g N2

4. Moles to Grams
Step 2 Write a mole–mole factor from the coefficients in the equation.
1 mole of N2 = 2 moles of NH3
1 mole N and 2 moles NH3
2 moles NH mole N2

5. Moles to Grams
Step 3 Convert moles of given to moles of needed using the mole–mole factor.
1.1 mole N2 x 2 moles NH3 = 2.2 moles of NH3
1 mole N2

6. Moles to Grams
Step 4 Convert moles of needed substance to grams using molar mass.
1 mole of NH3 = g of NH3
1 mole NH3 and g NH3
17.0 g NH mole NH3
2.2 moles NH3 x g NH3 = 37 g of NH3
1 mole NH3

7. Learning Check
How many grams of O2 are needed to produce 45.8 grams of
Fe2O3 in the following reaction?
4Fe(s) + 3O2(g) Fe2O3(s)
A g of O2
B g of O2
C g of O2

8. Solution How many grams of O2 are needed to produce 45.8 grams of
Fe2O3 in the following reaction?
4Fe(s) + 3O2(g) Fe2O3(s)
Step 1 Use molar mass to convert grams of given to moles.
1 mole of Fe2O3 = g of Fe2O3
1 mole Fe2O and g Fe2O3
159.7 g Fe2O mole Fe2O3
45.8 g Fe2O3 x 1 mole Fe2O3 = mole of Fe2O3
159.7 g Fe2O3

9. Solution
Step 2 Write a mole–mole factor from the coefficients in the equation.
3 moles of O2 = 2 moles of Fe2O3
3 moles O and moles Fe2O3
2 moles Fe2O moles O2
Step 3 Convert moles of given to moles of needed using the mole–mole factor.
0.287 mole Fe2O3 x 3 moles O = mole of O2
2 moles Fe2O3

10. Solution
Step 4 Convert moles of needed substance to grams using molar mass.
1 mole of O2 = g of O2
1 mole O2 and g O2
32.0 gO mole O2
0.430 mole O2 x g O2 = g of O2
1 mole O2
The answer is B, 13.8 g of O2.

11. Learning Check
The reaction between H2 and O2 produces 13.1 g of water.
How many grams of O2 reacted?
2H2(g) O2(g) H2O(g)
? g g

12. Solution 2H2(g) + O2(g) 2H2O(g) ? g 13.1 g
Step 1 Use molar mass to convert grams of given to moles.
1 mole of H2O = g of H2O
1 mole H2O and g H2O
18.0 g H2O mole H2O
13.1 g H2O x 1 mole H2O = mole of H2O
18.0 g H2O

13. Solution
Step 2 Write a mole–mole factor from the coefficients in the equation.
2 moles of H2O = 1 mole of O2
2 moles H2O and mole O2_
1 moles O moles H2O
Step 3 Convert moles of given to moles of needed using the mole–mole factor.
0.728 mole H2O x 1 mole O2 = mole of O2
2 moles H2O

14. Solution
Step 4 Convert moles of needed substance to grams using molar mass.
1 mole of O2 = g of O2
0.364 mole O2 x g O2 = g of O2
1 mole O2