Free Limiting and Excess Reactants MCQs with Answers

1. The limiting reactant in a chemical reaction is the one that

• A. is present in the largest amount by mass
• B. is completely used up first and so determines the amount of product formed
• C. has the highest molar mass
• D. remains unreacted at the end

Explanation: The reaction stops when one reactant runs out, so that reactant fixes the maximum yield no matter how much of the others is present. It must be identified by comparing moles against the balanced equation, not by comparing masses, because a small mass of a light reactant can be many moles. The reactant left over at the end is the excess reactant.

2. In the reaction N2 + 3H2 gives 2NH3, if 1 mole of nitrogen is mixed with 2 moles of hydrogen, the limiting reactant is

• A. nitrogen, because it has the larger molar mass
• B. neither, because both are used up exactly
• C. hydrogen, because 3 moles would be needed to react with 1 mole of nitrogen
• D. nitrogen, because fewer moles of it are present

Explanation: The equation demands three moles of hydrogen for every mole of nitrogen, so 1 mole of nitrogen would need 3 moles of hydrogen and only 2 are available. Hydrogen therefore runs out first and limits the yield to two thirds of a mole of ammonia. Choosing the reactant present in the smaller number of moles without checking the ratio is the standard mistake.

3. In the reaction 2H2 + O2 gives 2H2O, 4 moles of hydrogen are mixed with 3 moles of oxygen. The amount of the excess reactant left over is

• A. 2 moles of hydrogen
• B. 3 moles of oxygen
• C. nothing, both are exactly consumed
• D. 1 mole of oxygen

Explanation: Four moles of hydrogen require only two moles of oxygen, so hydrogen is limiting and 3 minus 2, that is 1 mole of oxygen, remains unreacted. Four moles of water are produced. The point of the calculation is that the excess is found by subtracting what was actually used from what was supplied, not by comparing the starting amounts directly.