In a reaction with 1:1 stoichiometry between A and B, you mix 8.0 g of A (M = 40 g/mol) and 2.0 g of B (M = 20 g/mol). Which reagent is limiting, and what is the theoretical yield if the product has molar mass 60 g/mol?

• A. A is limiting; theoretical yield = 8.0 g.
• B. B is limiting; theoretical yield = 12.0 g.
• C. B is limiting; theoretical yield = 6.0 g.
• D. A is limiting; theoretical yield = 3.0 g.

Answer: (C)

In a reaction with 1:1 stoichiometry, the limiting reagent is determined by which reactant provides fewer moles, because that one runs out first and sets the maximum amount of product possible.

Compute moles: A = 8.0 g ÷ 40 g/mol = 0.20 mol. B = 2.0 g ÷ 20 g/mol = 0.10 mol. Since they react in a 1:1 ratio, the smaller mole amount limits the reaction. The smaller amount corresponds to B, so B is the limiting reagent.

Theoretical yield comes from the limiting amount of product: 0.10 mol × 60 g/mol = 6.0 g.

Therefore, B is limiting and the theoretical yield is 6.0 g.