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Acids, bases and equilibrium

Written by Dr. Ayesha Nadeem (Chemistry)

11 MDCAT Chemistry questions with answers and explanations.

Printable worksheet with answer key

All questions with answers
  1. What is the pH of 0.01 M hydrochloric acid?

    • 1 12
    • 2 7
    • 3 1
    • 4 2
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    Correct answer: 2

    HCl is a strong acid, so [H+] = 0.01 = 10^-2 and pH = 2.

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  2. What is the pH of 0.001 M sodium hydroxide at 25 degrees C?

    • 1 11
    • 2 10
    • 3 3
    • 4 13
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    Correct answer: 11

    [OH-] = 10^-3, so pOH = 3 and pH = 14 - 3 = 11.

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  3. What is the conjugate base of H2SO4?

    • 1 SO4 2-
    • 2 H3SO4+
    • 3 OH-
    • 4 HSO4-
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    Correct answer: HSO4-

    When H2SO4 gives away one proton it becomes HSO4-, which is its conjugate base.

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  4. For N2 + 3H2 <-> 2NH3, increasing the pressure shifts the equilibrium:

    • 1 To the left
    • 2 To the right
    • 3 It does not change
    • 4 Only if a catalyst is added
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    Correct answer: To the right

    The forward reaction reduces 4 moles of gas to 2 moles. Higher pressure favours the side with fewer gas moles, so it shifts right.

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  5. A buffer solution is made of:

    • 1 A strong acid and a strong base
    • 2 Pure water and a salt
    • 3 Two salts of strong acids
    • 4 A weak acid and its salt
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    Correct answer: A weak acid and its salt

    A weak acid and its conjugate base resist large changes in pH when small amounts of acid or base are added.

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  6. What is the pOH of a solution whose pH is 4 at 25 degrees C?

    • 1 14
    • 2 4
    • 3 7
    • 4 10
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    Correct answer: 10

    pH + pOH = 14, so pOH = 14 - 4 = 10.

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  7. Which expression gives the solubility product Ksp of AgCl?

    • 1 [Ag+] x [Cl-]
    • 2 [Ag+] / [Cl-]
    • 3 [AgCl]
    • 4 [Ag+] + [Cl-]
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    Correct answer: [Ag+] x [Cl-]

    For a sparingly soluble salt, Ksp is the product of the ion concentrations at saturation.

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  8. A larger value of Ka means that the acid is:

    • 1 Weaker
    • 2 Stronger
    • 3 More basic
    • 4 Unable to ionise
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    Correct answer: Stronger

    Ka measures how far an acid ionises. A larger Ka means more ionisation, so a stronger acid.

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  9. The pH of a solution with a hydrogen ion concentration of 1 × 10⁻³ mol/dm³ is:

    • 1 11
    • 2 3
    • 3 −3
    • 4 7
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    Correct answer: 3

    pH = −log[H⁺] = −log(10⁻³) = 3.

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  10. A buffer solution is one that:

    • 1 Changes colour at its end point
    • 2 Resists a change in pH when small amounts of acid or base are added
    • 3 Has a pH of exactly 7
    • 4 Contains only a strong acid
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    Correct answer: Resists a change in pH when small amounts of acid or base are added

    A buffer contains a weak acid and its salt (or a weak base and its salt) and keeps the pH almost constant.

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  11. For the reaction N₂ + 3H₂ ⇌ 2NH₃ (exothermic), increasing the pressure shifts the equilibrium:

    • 1 Towards ammonia
    • 2 Towards nitrogen and hydrogen
    • 3 Not at all
    • 4 Towards the side with more moles of gas
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    Correct answer: Towards ammonia

    Four moles of gas on the left become two on the right, so higher pressure favours ammonia.

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