In a reaction with two reactants, the theoretical yield is determined by which reactant?

In a reaction with two reactants, the theoretical yield is determined by which reactant?

• A. The reactant in the greatest amount
• B. The limiting reactant
• C. The reactant with the highest molar mass
• D. The reactant that is most abundant

Answer: (B)

The amount of product you can make is determined by the limiting reactant—the one that is consumed first. Only the product formed from the limiting reactant can be produced, because once that reactant runs out the reaction cannot continue even if the other reactant is still present. To find the theoretical yield, use the balanced equation to convert moles of the limiting reactant into moles of product. This gives the maximum possible amount of product (assuming the reaction goes to completion). For example, in a reaction where 4 moles of hydrogen and 3 moles of oxygen react to form water (2 H2 + O2 → 2 H2O), hydrogen is the limiting reactant because you’d need 6 moles of H2 to use up all the O2, but only 4 moles are available. Those 4 moles of H2 can produce 4 moles of H2O, which sets the theoretical yield. The remaining oxygen can’t react without more hydrogen. The other options aren’t reliable predictors of yield: having more of one reactant, or it being the most abundant or having the highest molar mass, doesn’t guarantee it will determine the amount of product. That role is played by the reactant that runs out first.

The amount of product you can make is determined by the limiting reactant—the one that is consumed first. Only the product formed from the limiting reactant can be produced, because once that reactant runs out the reaction cannot continue even if the other reactant is still present.

To find the theoretical yield, use the balanced equation to convert moles of the limiting reactant into moles of product. This gives the maximum possible amount of product (assuming the reaction goes to completion).

For example, in a reaction where 4 moles of hydrogen and 3 moles of oxygen react to form water (2 H2 + O2 → 2 H2O), hydrogen is the limiting reactant because you’d need 6 moles of H2 to use up all the O2, but only 4 moles are available. Those 4 moles of H2 can produce 4 moles of H2O, which sets the theoretical yield. The remaining oxygen can’t react without more hydrogen.

The other options aren’t reliable predictors of yield: having more of one reactant, or it being the most abundant or having the highest molar mass, doesn’t guarantee it will determine the amount of product. That role is played by the reactant that runs out first.