1. Chapter 8 Chemical Quantities in Reactions
8.2 Mass Calculations for Reactions
Copyright © by Pearson Education, Inc. Publishing as Benjamin Cummings

2. Calculating the Mass of a Reactant
The reaction between H2 and O2 produces 13.1 g H2O.
How many grams of O2 reacted?
2H2(g) O2(g) 2H2O(g)
STEP 1 Given g H2O Need grams O2
STEP 2 Plan: g H2O mol H2O mol O g O2
STEP 3 1 mol O2 = 2 mol H2O mol H2O = g
1 mol O2 = g O2
STEP g H2O x 1 mol H2O x 1 mol O2 x g O g H2O mol H2O 1 mol O2
= 11.6 g O2

3. Learning Check How many grams of O2 are needed to produce
0.400 mol Fe2O3?
4Fe(s) O2(g) Fe2O3(s)
1) g O2
2) g O2
3) g O2

4. Solution 2) 19.2 g O2 mole factor molar mass
0.400 mol Fe2O3 x 3 mol O2 x g O mol Fe2O mol O2
= g O2

5. Learning Check 2C2H2(g) + 5O2(g) 4CO2(g) + 2H2O(g) 1) 88.6 g 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 CO2?
2C2H2(g) O2(g) CO2(g) + 2H2O(g)
1) g C2H2
2) g C2H2
3) g C2H2

6. Solution 3) 22.2 g C2H2 2C2H2(g) + 5O2(g) 4CO2(g) + 2H2O(g)
75.0 g CO2 x 1 mol CO2 x 2 mol C2H2 x g C2H2
44.01 g CO mol CO mol C2H2
= g C2H2