Limiting and Excess Reactants

Limiting and Excess Reactants MDCAT MCQs with answers

8 past paper MCQs on Limiting and Excess Reactants, from the Fundamental Concepts of Chemistry unit of MDCAT Chemistry. The year each question appeared is shown where known; tap “Show answer” for the correct option and a short solution.

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1.Consider the given reaction: N2 + 3H2 --> 2NH3 If 56 g of N2 reacts with 12 g of H2 and produces 51 g of NH3, what is the theoretical yield (TY) of NH3 and the percentage yield (PY) of the reaction? (Molar mass of N2 = 28 g/mol, H2 = 2 g/mol and NH3 = 17 g/mol) (SIBA MDCAT 2025)

  1. TY = 68 g and PY = 75%
  2. TY = 34 g and PY = 67%
  3. TY = 68 g and PY = 33%
  4. TY = 34 g and PY = 75%
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Correct answer: (a) TY = 68 g and PY = 75%

Two moles of nitrogen and six of hydrogen are exactly the required ratio, so four moles, $68\,\mathrm{g}$, are possible and $51/68$ is $75\%$.

2.50.0 g Mg is burnt with 32g of oxygen to form MgO, amount of excess reagent left is? (SIBA MDCAT 2025)

  1. 6g Mg
  2. 2g Mg
  3. 8g O2
  4. 16g O2
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Correct answer: (b) 2g Mg

One mole of oxygen needs two moles, $48\,\mathrm{g}$, of magnesium, so $2\,\mathrm{g}$ of magnesium is left over.

3.Ca + S → CaS. If 2.0g of Calcium and 4.0g of Sulphur are available, amount of product formed: (NUMS MDCAT 2024)

  1. 4.6g
  2. 1.6g
  3. 2.6g
  4. 3.6g
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Correct answer: (d) 3.6g

$2.0\,\mathrm{g}$ is $0.05$ mole of calcium, the limiting reactant, so $0.05\times72$, that is $3.6\,\mathrm{g}$, of the sulphide is formed.

4.The limiting reactant in a particular chemical reaction is the one which: (PMC MDCAT 2022)

  1. Is consumed later in the reaction
  2. Is present in excess in the reaction
  3. Gives the least amount of product
  4. Gives the largest amount of product
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Correct answer: (c) Gives the least amount of product

The limiting reactant is used up first and so fixes how much product can be made.

5.The amount of reactant according to the balanced chemical equation is called as? (PMC MDCAT 2022)

  1. Excess amount
  2. Actual amount
  3. Limiting amount
  4. Stoichiometric amount
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Correct answer: (d) Stoichiometric amount

The amount the equation calls for is the stoichiometric amount.

6.The ________ reagent is the reactant that gets consumed first in a chemical reaction and therefore limits how much product can be formed. (PMC MDCAT 2022)

  1. excess
  2. limiting
  3. extent
  4. none of the above
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Correct answer: (b) limiting

The reactant used up first is the limiting reagent.

7.Which one of the following is calculated from balanced equation during the determination of limiting reactant? (PMC MDCAT 2022)

  1. Amount of reactants present
  2. Amount of limiting reactant that produces·the product
  3. Amount of product expected to form from each reactant
  4. Amountof excess reactant
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Correct answer: (c) Amount of product expected to form from each reactant

Each reactant is converted to the product it could give, and the smaller answer settles which is limiting.

8.In the process of iron rusting, which of the following is the limiting reactant? (PMC MDCAT 2022)

  1. Iron
  2. Oxygen
  3. Air
  4. Water
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Correct answer: (a) Iron

Air supplies oxygen and water without limit, so it is the iron that runs out.

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