Chemical Equilibrium

Chemical Equilibrium MDCAT MCQs with answers

75 past paper MCQs from Chemistry unit 6, Chemical Equilibrium, across 6 topics. They are arranged topic by topic; bigger topics show a sample here and link to their full set and to study notes. Tap “Show answer” for the correct option and a short solution.

Practise as a quiz

Chemical Equilibrium

1.At equilibrium the concentrations of the reactants and the products become: (MDCAT 2020)

  1. zero
  2. equal
  3. constant
  4. infinite
Show answer

Correct answer: (c) constant

The two opposing rates are equal, so the concentrations stop changing, though they need not be equal to each other.

2.Chemical equilibrium is: (MDCAT 2023)

  1. the end of the reaction
  2. the complete consumption of the reactants
  3. the complete consumption of the products
  4. a state at which there is no net concentration change
Show answer

Correct answer: (d) a state at which there is no net concentration change

Both reactions continue at equal rates, so the concentrations stop changing without the reaction stopping.

3.If the equilibrium expression of a reversible reaction is [C]2/[A][B] The balanced chemical equation should be (SZABMU MDCAT 2025)

  1. C=A+B
  2. 2C=A+B
  3. A+B=C
  4. A + B = 2C
Show answer

Correct answer: (d) A + B = 2C

The powers in the expression are the coefficients, so two moles of C are formed from one of A and one of B.

All 15 Chemical Equilibrium MCQs → Chemical Equilibrium notes →

Le Chatelier's Principle

4.For a gaseous reaction, an increase in pressure will shift the equilibrium in the direction of: (MDCAT 2020)

  1. decreased concentration
  2. increased concentration
  3. decreased volume
  4. increased volume
Show answer

Correct answer: (c) decreased volume

The system relieves the pressure by moving to the side with fewer gas molecules, which occupies less volume.

5.In the reaction $\mathrm{H_2+CO_2\rightleftharpoons H_2O+CO}$, a decrease in the concentration of $\mathrm{CO_2}$ will shift the equilibrium: (MDCAT 2020)

  1. towards the left
  2. towards the right
  3. nothing happens to the equilibrium
  4. towards both directions
Show answer

Correct answer: (a) towards the left

Taking a reactant away makes the reverse reaction the faster of the two, so the equilibrium moves back to the left.

6.When the numbers of moles of reactants and products are equal in a reversible gaseous reaction, which parameter would not affect the equilibrium? (MDCAT 2020)

  1. temperature
  2. pressure
  3. concentration
  4. catalyst
Show answer

Correct answer: (b) pressure

With no change in the number of gas molecules there is no volume change for the pressure to act on.

All 15 Le Chatelier's Principle MCQs → Le Chatelier's Principle notes →

Solubility Product

7.Precipitation occurs if the ionic concentration is: (MDCAT 2022)

  1. greater than $K_{sp}$
  2. less than $K_{sp}$
  3. equal to $K_{sp}$
  4. present in any amount
Show answer

Correct answer: (a) greater than $K_{sp}$

The solution can hold only as much as $K_{sp}$ allows, so anything above that comes out as solid.

8.The solubility of PbS at 25°C is 4.0 × 10−28. Then ionic concentration will be: (NUMS MDCAT 2024)

  1. 4x10−14
  2. 2x10−14
  3. 1x10−14
  4. 3.0x10−10
Show answer

Correct answer: (b) 2x10−14

For a one to one salt $K_{sp}=s^2$, so $s=\sqrt{4.0\times10^{-28}}$, that is $2\times10^{-14}$.

9.Q.84 The precipitation occurs if the ionic concentration is: (UHS MDCAT 2023)

  1. Less than ksp
  2. More than ksp
  3. Equal to ksp
  4. Present in any amount
Show answer

Correct answer: (b) More than ksp

A solid comes down once the ionic product exceeds the solubility product.

All 9 Solubility Product MCQs → Solubility Product notes →

Common Ion Effect

10.The decrease in solubility of a salt in a solution that already contains an ion common to that salt is known as: (MDCAT 2022)

  1. Le Chatelier's principle
  2. solubility product
  3. common ion effect
  4. $K_{sp}$
Show answer

Correct answer: (c) common ion effect

The added common ion shifts the dissolution equilibrium back towards the solid.

11.Q.83 The decrease in solubility of the salt in a solution that already contains an ion common to that salt is known as: (UHS MDCAT 2023)

  1. Le Chatelier's principle
  2. Solubility Product
  3. Common ion effect
  4. Ksp
Show answer

Correct answer: (c) Common ion effect

Adding a common ion drives the dissolution equilibrium back, which is the common ion effect.

12.When Hydrogen Chloride gas passes through saturated brine, it increases the concentration of: (PMC MDCAT 2022)

  1. Sodium ion
  2. Hydrogen ion
  3. Calcium ion
  4. Chloride ion
Show answer

Correct answer: (d) Chloride ion

The added chloride is the ion common to the salt, so it drives the sodium chloride out of solution.

All 7 Common Ion Effect MCQs → Common Ion Effect notes →

Buffer Solutions

13.A buffer solution is not formed by: (MDCAT 2020)

  1. $\mathrm{NH_4OH+NH_4Cl}$
  2. $\mathrm{CH_3COOH+CH_3COONa}$
  3. $\mathrm{C_2H_5COOH+C_2H_5COONa}$
  4. $\mathrm{HCl+NaCl}$
Show answer

Correct answer: (d) $\mathrm{HCl+NaCl}$

A buffer needs a weak acid or base with its own salt; HCl is a strong acid, so this mixture has no reserve to soak up added acid or alkali.

14.Which of the following is basic buffer? (UHS MDCAT 2025)

  1. NH4OH/NH4Cl
  2. NaOH/NaCl
  3. NaOH/HCl
  4. H2CO3/NaHCO3
Show answer

Correct answer: (a) NH4OH/NH4Cl

A basic buffer is a weak base with the salt of that base, here ammonium hydroxide with ammonium chloride.

15.What happens when H+ is added to ammonium hydroxide and ammonium chloride buffer? (UHS MDCAT 2025)

  1. more ammonium hydroxide is formed
  2. reaction will move reverse
  3. reaction will move forward
  4. no effect on equilibrium
Show answer

Correct answer: (b) reaction will move reverse

The added protons are taken up by the ammonium hydroxide, driving the ionization back and holding the pH.

All 16 Buffer Solutions MCQs → Buffer Solutions notes →

Haber's Process

16.Identify the CORRECT conditions required for the maximum yield of ammonia by Haber's process: (MDCAT 2024)

  1. high pressure, low temperature and continual removal of ammonia
  2. low pressure, low temperature and continual removal of ammonia
  3. high pressure, high temperature and continual removal of ammonia
  4. high pressure, low temperature and continual addition of ammonia
Show answer

Correct answer: (a) high pressure, low temperature and continual removal of ammonia

The forward reaction is exothermic and goes with a fall in the number of moles, so pressure helps, heat hurts, and taking the ammonia out keeps pulling it forward.

17.According to Le Chatelier's principle, in Haber's process yield of ammonia can be increased by: (SZABMU MDCAT 2025)

  1. decreasing pressure and increasing temperature
  2. increasing pressure and decreasing temperature
  3. increasing volume and decreasing temperature
  4. decreasing concentration of H2
Show answer

Correct answer: (b) increasing pressure and decreasing temperature

The forward reaction is exothermic and goes from four molecules to two, so high pressure and low temperature favour it.

18.Identify the CORRECT option required for the maximum yield of ammonia by Haber's process: (UHS MDCAT 2024)

  1. High pressure low temperature continual removal of ammonia
  2. Low pressure low temperature continual removal of ammonia
  3. High pressure high temperature continual removal of ammonia
  4. High pressure low temperature continual addition of ammonia
Show answer

Correct answer: (a) High pressure low temperature continual removal of ammonia

The forward reaction is exothermic and contracts in volume, and taking the ammonia away keeps pulling it forward.

All 13 Haber's Process MCQs → Haber's Process notes →

More MDCAT Chemistry units