Work out how much product a reaction really makes, and how much of the reactants end up in the product you want. Every question uses real balanced equations and the rules below.
The rules
- Percentage yield = actual mass of product ÷ theoretical mass × 100. Actual on top. The answer can never be more than 100 %.
- Theoretical yield comes from the balanced equation. Find moles of reactant (mass ÷ Mr), use the ratio from the equation, then mass of product = moles × Mr. Higher
- Yields are less than 100 % because the reaction may be reversible, some product is lost when it is separated, and side reactions can happen.
- Atom economy = Mr of desired product ÷ total Mr of all reactants × 100. The bottom line is the reactants, not the products.
- Multiply each Mr by its balancing number from the equation, for the reactants and for the desired product. 3CO means 3 × 28 = 84.
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