Quick questions on Percentage yield and atom economy: WACE Year 12 Chemistry

$$\text{percentage yield} = \frac{\text{actual yield}}{\text{theoretical yield}} \times 100$$

$$\text{atom economy} = \frac{\text{molar mass of desired product}}{\text{total molar mass of all products}} \times 100$$

Almost every percentage-yield calculation hinges on correctly identifying the limiting reagent, because the theoretical yield is set by whichever reactant runs out first. The reliable method is to convert the given masses of each reactant to moles, then divide each by its coefficient in the balanced equation; the smallest value identifies the limiting reagent. The theoretical yield of product is then found from that reagent using the mole ratio, converted back to mass with the molar mass. A common trap is to assume the reactant present in the smaller mass is limiting, but a low-molar-mass reactant can supply more moles, so always compare in moles, not grams.