Chemical Calculations Lesson # 7 – Percentage Yield

Introduction In experiments, we can predict how much product we will have if we know the limiting reagent, however it is sometimes difficult to get 100% of the product, so where does it go? Residue remaining on glassware and apparatus Human error Evaporation Wet vs. dry weight (moisture content)

Yield Definitions The theoretical yield is the maximum amount of product that can be formed given the amount of the reactants. We calculated this in the previous lesson once we found the limiting reagent. The actual yield is the amount of product that actually forms when the reaction is carried out, and is usually less than the theoretical yield. The only time it might be more is when the product is retaining excess moisture. The percent yield is the ratio of the actual yield to the theoretical yield, expressed as a percent. % Yield = Actual Yield x 100% Theoretical Yield

Example 1 If 20.4 g of calcium carbonate is heated, determine the theoretical yield of calcium oxide, and then the percent yield, if 10.6 g are actually produced in the lab. CaCO3 → CaO + CO2

Example 2 Suppose 7.00 g of AgNO3 is added to a solution which contains an excess of KBr. If 7.32 g of AgBr is obtained in the lab, what is the % yield? AgNO3 + KBr → AgBr + KNO3