Analyze the chemical equation and write down the given information. When doing a percent yield calculation, you are typically given the amount of the reactants as well as the amount of product that was obtained. For example, if the chemical equation is C2H4 + HBr gives you C2H5Br, then the reactants are ethene, C2H4, and hydrogen bromide, HBr. You may have started with 9.00 g of C2H4 and 9.00 g of HBr. The product is bromoethane, C2H5Br. The actual yield was 10.0 g of
C2H5Br.
Determine the molar mass of the reactants and products. Examine the elements that compose each compound, find their masses in the periodic table, and then add them together. For example, for C2H4, one mole of carbon has 12.01 g/mol. One mole of hydrogen has 1.008 g/mol. Since C2H4 has two moles of carbon and four moles of hydrogen, multiply 12.01 g/mol by 2, and multiply 1.008 g/mol by four. Adding these masses together gives you 28.05 g/mol in C2H4. The molar mass of HBr is 80.91 g/mol. The molar mass for C2H5Br is 108.96 g/mol.
Determine which reactant is the limiting reagent. The limiting reagent is the reactant that produces the smaller amount of moles of product. You can use dimensional analysis for this process. For example, for C2H4, the first ratio is 9.0 g of C2H4 over 1. The second ratio is 1 mole of C2H4 over 28.05 g of C2H4. The third ratio is 1 mole of C2H5Br over 1 mole of C2H4. Multiplying these ratios gives you 0.321 moles of C2H5Br. For HBr, the first ratio is 9.0 g HBr over 1. The second ratio is 1 mole of HBr over 80.91 g of HBr. The third ratio is 1 mole of C2H5Br over 1 mole of HBr. Multiplying these ratios gives you 0.111 moles of C2H5Br. Since HBR gives you a smaller amount of product, HBr is the limiting reagent.
Find the amount of grams of product produced by the limiting reagent. For example, since 9.0 g of HBr produces 0.111 moles of C2H5Br, then you can use dimensional analysis to find the number of grams of C2H5Br. The first ratio is 0.111 moles of C2H5Br over 1. The second ratio is 108.96 g of C2H5Br over 1 mole of C2H5Br. This gives an answer of 12.1 g of C2H5Br. Therefore, the theoretical yield of C2H5Br is 12.1 grams.
Write down the formula for the percent yield, and put in the proper values. The formula for percent yield is (actual yield / theoretical yield) x 100. For example, if the actual yield is 10.0 g of C2H5Br, and the theoretical yield is 12.1 g/mol, then the percent yield is 82.6 percent.