1. Chapter 6 Chemical Reactions and Quantities
Mass Calculations for Reactions

2. Guide to Calculating the Masses of Reactants and Products

3. Example of Converting Moles to Grams
Suppose we want to determine the mass (g) of NH3
that can form from 2.50 moles of N2.
N2(g) + 3H2(g) NH3(g)
STEP 1 Given: moles of N2
Need: mass (g) of NH3
STEP 2 Plan:
moles of N moles of NH grams of NH3

4. Moles to Grams STEP 3 Write conversions factors:
1 mole of N2 = 2 moles of NH3
1 mole N2 and 2 moles NH3
2 moles NH mole N2
1 mole of NH3 = g of NH3
1 mole of NH3 and g of NH3
17.0 g of NH mole of NH3
STEP 4 Setup to calculate g of NH3:
2.50 moles N2 x 2 moles NH3 x g NH3
1 mole N mole NH3
= g of NH3

5. Learning Check How many grams of O2 are needed to produce
0.400 mole of Fe2O3 in the following reaction?
4Fe(s) + 3O2(g) Fe2O3(s)
1) g of O2
2) g of O2
3) g of O2

6. Solution 2) 19.2 g of O2 STEP 1 Given: 0.400 moles of Fe2O3
Need: mass (g) of O2
STEP 2 Write a plan to calculate grams of O2:
moles of Fe2O moles of O grams of O2
STEP 3 Write conversion factors:
3 moles of O2 = 2 moles of Fe2O3
3 moles O2 and 2 moles Fe2O3
2 moles Fe2O moles O2

7. Solution (continued) STEP 3 (continued) 1 mole of O2 = 32.0 g of O2
1 mole O2 and g O2
32.0 g O mole O2
STEP 4 The setup to calculate g of O2 is
0.400 mole Fe2O3 x 3 moles O2 x g O2 = 19.2 g of O moles Fe2O mole O2
mole-mole factor molar mass

8. Learning Check 2C2H2(g) + 5O2(g) 4CO2(g) + 2H2O(g) 1) 88.6 g of C2H2
Acetylene gas, C2H2, burns in the oxyacetylene torch for welding. How many grams of C2H2 are burned if the reaction produces 75.0 g of CO2?
2C2H2(g) + 5O2(g) CO2(g) + 2H2O(g)
1) g of C2H2
2) g of C2H2
3) g of C2H2

9. Solution g of CO2 moles of moles of g of C2H2 CO2 C2H2
STEP 1 Given: g of CO2
Need: mass (g) of C2H2
STEP 2 Write a plan to calculate grams of C2H2:
g of CO moles of moles of g of C2H CO C2H2
STEP 3 Write conversion factors:
1 mole of CO = g of CO2
1 mole CO2 and g CO2
44.0 g CO mole CO2

10. Solution (continued) STEP 3 Write conversion factors (continued):
4 moles of CO2 = 2 moles of C2H2
4 moles CO2 and 2 moles C2H2
2 moles C2H moles CO2
1 mole of C2H2 = g of C2H2
1 mole C2H2 and g C2H2
26.0 g CO mole C2H2
STEP 4 Setup to calculate g of O2:
75.0 g CO2 x 1 mole CO2 x 2 moles C2H2 x g C2H2
44.0 g CO moles CO mole C2H2
= g C2H2 (3)

11. Learning Check
How many grams H2O are produced when 35.8 g of C3H8 react by the following equation?
C3H8(g) + 5O2(g) CO2(g) + 4H2O(g)
g of H2O
g of H2O
g of H2O

12. Solution 2) 58.4 g H2O STEP 1 Given: 38.5 g of C3H8
Need: mass (g) of H2O
STEP 2 Write a plan to calculate grams of H2 O:
g of C3H moles of moles of g of H2O C3H H2O
STEP 3 Write conversion factors:
1 mole of C3H8 = 4 moles of H2O
4 moles H2O and 1 mole C3H8
1 mole C3H moles H2O

13. Solution (continued) STEP 3 Write conversion factors (continued):
1 mole of C3H8 = g of C3H8
1 mole C3H and g C3H8
44.1 g C3H mole C3H8
1 mole of C2H2 = g of C2H2
1 mole H2O and g H2O
18.0 g H2O mole H2O
STEP 4 Setup to calculate g of H2O:
35.8 g C3H8 x 1 mole C3H8 x 4 moles H2O x 18.0 g H2O
44.1 g C3H mole C3H mole H2O
= g of H2O (2)