Chapter 5 Chemical Quantities and Reactions 5.7 Mass Calculations for Reactions

Mass Calculations in Equations

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) 2NH3(g) Step 1 Use molar mass to convert grams of given to moles. 1 mole of N2 = 28.0 g of N2 1 mole N2 and 28.0 g N2 28.0 g N2 1 mole N2 32 g N2 x 1 mole N2 = 1.1 mole of N2 28.0 g N2

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 N2 and 2 moles NH3 2 moles NH3 1 mole N2

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

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

Learning Check How many grams of O2 are needed to produce 45.8 grams of Fe2O3 in the following reaction? 4Fe(s) + 3O2(g) 2Fe2O3(s) A. 38.4 g of O2 B. 13.8 g of O2 C. 1.38 g of O2

Solution How many grams of O2 are needed to produce 45.8 grams of Fe2O3 in the following reaction? 4Fe(s) + 3O2(g) 2Fe2O3(s) Step 1 Use molar mass to convert grams of given to moles. 1 mole of Fe2O3 = 159.7 g of Fe2O3 1 mole Fe2O3 and 159.7 g Fe2O3 159.7 g Fe2O3 1 mole Fe2O3 45.8 g Fe2O3 x 1 mole Fe2O3 = 0.287 mole of Fe2O3 159.7 g Fe2O3

Solution Step 2 Write a mole–mole factor from the coefficients in the equation. 3 moles of O2 = 2 moles of Fe2O3 3 moles O2 and 2 moles Fe2O3 2 moles Fe2O3 3 moles O2 Step 3 Convert moles of given to moles of needed using the mole–mole factor. 0.287 mole Fe2O3 x 3 moles O2 = 0.430 mole of O2 2 moles Fe2O3

Solution Step 4 Convert moles of needed substance to grams using molar mass. 1 mole of O2 = 32.0 g of O2 1 mole O2 and 32.0 g O2 32.0 gO2 1 mole O2 0.430 mole O2 x 32.0 g O2 = 13.8 g of O2 1 mole O2 The answer is B, 13.8 g of O2.

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

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 = 18.0 g of H2O 1 mole H2O and 18.0 g H2O 18.0 g H2O 1 mole H2O 13.1 g H2O x 1 mole H2O = 0.728 mole of H2O 18.0 g H2O

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 1 mole O2_ 1 moles O2 2 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 = 0.364 mole of O2 2 moles H2O

Solution Step 4 Convert moles of needed substance to grams using molar mass. 1 mole of O2 = 32.0 g of O2 0.364 mole O2 x 32.0 g O2 = 11.6 g of O2 1 mole O2