1. Chapter 9 Chemical Quantities in Reactions
9.2 Mass Calculations for Reactions
In a combustion reaction, acetylene (C2H2) burns with oxygen to produce carbon dioxide and water.

2. Calculating the Masses of Reactants and Products

3. Example of Converting Moles to Grams of Product
Suppose we want to determine the mass (g) of NH3
that can form from 2.50 mol of N2.
N2(g) + 3H2(g) NH3(g)
STEP 1 Use molar mass to convert grams of given to moles (if necessary).
Given mol of N Need grams of NH3
STEP 2 Write a mole-mole factor from the coefficients in the equation.
1 mol of N2 = 2 mol of NH3
1 mol N2 and 2 mol NH3
2 mol NH mol N2

4. Converting Moles to Grams (continued)
STEP 3 Convert moles of given to moles of needed substance using the mole-mole factor.
2.50 mol N2 x 2 mol NH3 = 5.00 mol of NH3
1 mol N2
STEP 4 Convert moles of needed substances to grams using molar mass.
1 mol of NH3 = g of NH3
1 mol NH3 and g NH3
17.0 g NH mol NH3
5.00 mol NH3 x g NH3 = g of NH3
1 mol NH3

5. Learning Check 2C2H2(g) + 5O2(g) 4CO2(g) + 2H2O(g) 1) 88.4 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 of CO2?
2C2H2(g) O2(g) CO2(g) + 2H2O(g)
1) g C2H2
2) g C2H2
3) g C2H2

6. Solution
STEP 1 Use molar mass to convert grams of given to moles (if necessary).
Given g of CO2 Need grams of C2H2
1 mol of CO = g of CO2
1 mol CO and g CO2
44.01 g CO mol CO2
75.0 g CO2 x 1 mol CO2 = 1.70 mol of CO2
44.01 g CO2 6

7. Solution (continued)
STEP 2 Write a mole-mole factor from the coefficients in the equation.
2 mol of C2H2 = 4 mol of CO2
2 mol C2H2 and 4 mol CO2
4 mol CO mol C2H2
STEP 3 Convert moles of given to moles of needed substance using the mole-mole factor.
1.70 mol CO2 x 2 mol C2H2 = mol of C2H2
4 mol CO2 7

8. Solution (continued)
STEP 4 Convert moles of needed substances to grams using molar mass.
1 mol of C2H2 = g of C2H2
26.04 g C2H2 and 1 mol C2H2
1 mol C2H g C2H2
0.850 mol C2H2 x g C2H2 = g of C2H2
1 mol C2H2 8