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Limiting Reagent Questions

A reagent, also known as an analytical reagent, is a substance or compound that is added to a system to initiate or test a chemical reaction. The terms reactant and reagent are frequently used interchangeably, but reactant refers to a substance consumed during a chemical reaction.

Definition: The limiting reactant (or limiting reagent) is the reactant that is consumed first in a chemical reaction, limiting the amount of product that can be formed. There are numerous methods for determining the limiting reactant, but they all rely on mole ratios from the balanced chemical equation.

The theoretical yield is the amount of product that can be formed based on the limiting reactant. In practise, the actual yield, or amount of product collected, is almost always less than the theoretical yield. The actual yield is typically expressed as a percentage yield, indicating what percentage of the theoretical yield was obtained.

Limiting Reagent Chemistry Questions with Solutions

Q1. We can calculate the limiting reagent in a reaction by many factors, but which of the factors cannot help to determine the limiting reactant:

  1. Number of moles
  2. Mass given
  3. Volume given
  4. Pressure given

Correct Answer: (d) Pressure given

Q2. Silicon nitride (Si3N4) is made by combining Si and nitrogen gas (N2) at a high temperature. How much (in g) Si is needed to react with an excess of nitrogen gas to prepare 125 g of silicon nitride if the percent yield of the reaction is 95.0%?

  1. 89 g
  2. 79.1
  3. 75 g
  4. 90.8 g

Correct Answer: (b) 79.1 g

Q3. What is a limiting reactant?

  1. The reactant that is used up first and prevents more product from being made.
  2. The reactant that makes the product.
  3. The reactant that is used up last and prevents more product from being made
  4. The substance that is in excess that doesn’t get used up as a reactant.

Correct Answer: (a) The reactant that is used up first and prevents more product from being made.

Q4. In the following reaction:

4NH3(g) + 5O2 (g) → 4NO (g) + 6H2O (l)

When 1 mole ammonia and 1 mole of O2 are mixed, then the number of moles of NO formed will

be:

  1. 0.8
  2. 0.7
  3. 0.6
  4. 0.5

Correct Answer: (a) 0.8 mole

Q5. 10 g of marble(calcium carbonate) was added to 15 g of dilute hydrochloric acid, product formed is calcium chloride, water and carbon dioxide. Which of the following is true?

  1. Hydrochloric acid is the limiting reactant
  2. Hydrochloric acid is the excess reactant
  3. Calcium carbonate is the excess reactant
  4. None of the above

Correct Answer: (b) Hydrochloric acid is the excess reactant

Q6. What is a Limiting reagent?

Answer. A limiting reagent is a reactant that occurs in lower concentrations in a reaction. When it is consumed, the reaction will stop, regardless of the amount of reactant present in the reaction. It restricts the amount of product produced. In other words, it determines the magnitude of the reaction.

Q7. NO2 is formed when 0.740 g of O3 reacts with 0.670 g of NO. What is the limiting reagent?

Answer. O3 + NO → O2 +NO2

1 mole of O3 reacts with 1 mole of NO.

0.74 g O3 = 0.74 / 48 = 0.0154 mol O3

0.67 g NO = 0.67 / 30 = 0.0223 mol NO

O3 is the limiting reagent and NO is in excess.

Q8. How do you determine which product is the limiting one?

Answer. Compare the calculated amount of a reactant to the actual amount available to determine which reactant is the limiting one. If more is needed than what is available, it is the limiting reactant.

Q9. If 4.95 g of ethylene (C2H4) are combusted with 3.25 g of oxygen. What is the limiting reagent?

Answer. The limiting reagent would be O2.

Q10. Calculate the limiting reagent in 2H2 + O2 → 2H2O

Answer. Given 1 mol of hydrogen and 1 mol of oxygen in the reaction:

2H2 + O2 → 2H2O

The limiting reactant would be hydrogen because the reaction uses up hydrogen twice as fast as oxygen.

Q11. 50.0 kg of N2 (g) and 10.0 kg of H2 (g) are mixed to produce NH3 (g).

Identify the limiting reagent in the production of NH3 in this situation.

Answer.

N2 + 3H2 ⇆ 2NH3

1 mol N2 (g) requires 3 mol H2 (g), for the reaction.

Hence, for 17.86 ×102 mol of N2, the moles of H2 (g) required would be 17.86 ×102 mol N2 × (3 mol H2 (g)) / (1 mol N2 (g)

= 5.36 ×103 mol H2

But we have only 4.96×103 mol H2. Hence, dihydrogen is the limiting reagent

Q12. What is the greatest amount of MgO (i moles) that can be made with 7.8 moles of Mg and 4.7 moles of O2? Which is the limiting reagent?

Answer. 2Mg + O2 → 2MgO

For 7.8 moles of Mg = (1 moles of O2 / 2 moles of Mg) × 7.8 mole Mg = 3.9 moles of O2

For 4.7 moles of O2 = (2moles of Mg / 1 mole of O2) × 4.7 moles O2 = 9.4 moles Mg.

Therefore, Mg is the limiting reagent.

Q13. Calculate the amount of C formed in the reaction 2A + 4B ⟶ 3C + 4D when 5 moles of A react with 6 moles of B.

Answer. The balanced reaction is 2A + 4B ⟶ 3C + 4D

Moles given is 5 moles of A and 6 moles of B

Ratio of moles given and stoichiometry coefficients:

For A: 5/2 = 2.5

For B: 6/4 = 1.5

Since 1.5 is less than 2.5, B is the limiting reagent because it will be used first.

Therefore, the moles produced of C = ¾ = 4.5 mol.

Q14 What would be the limiting reagent if 75 grams of C2H3Br3 reacted with 50.0 grams of O2 in the following reaction:

4C2H3Br3 + 11O2 → 8CO2 + 6H2O + 6Br2.

Answer. Conversion to moles

75 g × (1mole / 266.72 g) = 0.28 mole C2H3Br3

50 g × (1mole /32 g) = 1.56 mol O2

To calculate how much C2H3Br3 would be required if all the O2 is used up:

1.56 mol O2 × (4 mol C2H3Br3 /11 mol O2) = 0.567 mol C2H3Br3

This shows that 0.567 mol C2H3Br3 is needed to react with all of the oxygen. C2H3Br3 is the limiting reagent because there is only 0.28 mol of it present.

Q15. What would be the limiting reagent if 80.0 grams of Na2O2 reacted with 30.0 grams of H2O in the reaction?

2Na2O2 + 2H2O → 4NaOH + O2

Answer. 80 g Na2O2 × (1 mol Na2O2/77.98 g Na2O2) × (4moles NaOH / 2moles Na2O2) = 2.06 moles NaOH

30 g H2O × (1 mol H2O/18g H2O) × (4moles NaOH / 2moles Na2O2) = 3.33 moles NaOH

Since Na2O produces less NaOH than H2O, it is the limiting reagent.

Practise Questions on Limiting Reagent

Q1. The reactant which is not consumed completely in the reaction is _____.

Q2. In a reaction: – A + B2 → AB2

Identify the limiting reagent, if any, in the following reaction mixtures.

(i) 300 atoms of A + 200 molecules of B

(ii) 2 mol A + 3 mol B

(iii) 100 atoms of A + 100 molecules of B

(iv) 5 mol A + 2.5 mol B

(v) 2.5 mol A + 5 mol B

Q3. The reactant which is entirely consumed in the reaction is known as the limiting reagent. In the reaction 2A + 4B⟶ 3C + 4D, if 5 moles of A react with 6 moles of B then, which is the limiting reagent?

  1. A
  2. B
  3. C
  4. D

Q4. If you have an actual yield of 29.3 grams of product and the theoretical yield is 35.0 grams, what is your percent yield?

Q5. 6 g of H2 reacts with 14 g of N2 to form NH3 until the reaction consumes the limiting reagent completely. The amount of another reactant remaining in g is:

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