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.

Calculating the Masses of Reactants and Products

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) 2NH3(g) STEP 1 Use molar mass to convert grams of given to moles (if necessary). Given 2.50 mol of N2 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 NH3 1 mol N2

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 = 17.0 g of NH3 1 mol NH3 and 17.0 g NH3 17.0 g NH3 1 mol NH3 5.00 mol NH3 x 17.0 g NH3 = 85.0 g of NH3 1 mol NH3

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) + 5O2(g) 4CO2(g) + 2H2O(g) 1) 88.4 g C2H2 2) 44.2 g C2H2 3) 22.1 g C2H2

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

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 CO2 2 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 = 0.850 mol of C2H2 4 mol CO2 7

Solution (continued) STEP 4 Convert moles of needed substances to grams using molar mass. 1 mol of C2H2 = 26.04 g of C2H2 26.04 g C2H2 and 1 mol C2H2 1 mol C2H2 26.04 g C2H2 0.850 mol C2H2 x 26.04 g C2H2 = 22.1 g of C2H2 1 mol C2H2 8