Calculate percentage yield and percentage purity
Yield is the amount of product you get from a reaction. Find the theoretical yield first by determining how much of the product should be formed based on moles of the reactant calculated. The actual yield is the amount of product that is formed when the actual reaction is carried out in the laboratory.
Formula for find percentage yield = (actual yield ÷ theoretical yield) x 100%
Example: 10g of calcium carbonate is heated strongly to form 2.8g of calcium oxide. Find the percentage yield of calcium oxide.
Balanced chemical equation: CaCO3 → CaO + CO2
Step 1: find the mass of calcium oxide that can be obtained from 100g of calcium carbonate. 2.8g of CaO is the actual mass.
Mole of calcium carbonate = mass ÷ Mr = 10 ÷ 100 = 0.1 mol
From the balanced chemical equation, 1 mole of CaCO3 produce 1 mole of CaO.
mole of CaCO3 produce 0.1 mole of CaO.Theoretical mass of CaO = mole x Mr = 0.1 x 56 = 5.6g
Step 2: Put in the calculated values in the formula for percentage yield.
Percentage yield = (2.8 ÷ 5.6) x 100% = 50.0%
Purity is very important as impurities can have harmful effects in a drug or medicine. Impurities can also interfere reactions of the pure substance involved. Find the pure mass of reactant asked in the question first by determining the moles of the products or other reactants reacted.
Formula for find percentage purity = (amount of pure substance ÷ amount of impure substance) x 100%
Example: 10g of impure carbon is heated strongly with excess oxygen. Volume of carbon dioxide collected is 12dm3. Find the percentage purity of carbon.
Balanced chemical equation: C + O2 → CO2
Step 1: find the mass of carbon that reacted to form 1.2dm3 of carbon dioxide. 10g is the mass of impure carbon.
Mole of carbon dioxide = 12/24 = 0.5 mol
From the balanced chemical equation, 1 mole of carbon dioxide is produced from 1 mole of carbon.
so 0.5 mole of CO2 is formed from 0.5 mole of C.
Pure mass of carbon = mole x Ar = 0.5 x 12 = 6g
Step 2: Put in the calculated values in the formula for percentage purity.
Percentage purity = (6 ÷ 10) x 100% = 60.0%