Ionic Equilibrium - JEE Mains Previous Year Questions

Here are 10 previous year questions (PYQs) from the topic of Ionic Equilibrium to help you prepare for JEE Mains:

1. Q1: The pH at which Mg(OH)₂ [K_{sp} = 1 \times 10^{-11}] begins to precipitate from a solution containing 0.10 M Mg²⁺ ions is ______.

   Solution:

   \[ [\text{Mg}^{2+}][\text{OH}^-]^2 = K_{sp} \] \[ [0.1][\text{OH}^-]^2 = 1 \times 10^{-11} \] \[ [\text{OH}^-] = 10^{-5} \, \text{M} \] \[ \text{pOH} = -\log(10^{-5}) = 5 \] \[ \text{pH} = 14 - \text{pOH} = 14 - 5 = 9 \] Answer: 9
2. Q2: Given below are two statements:

   Statement (I): A buffer solution is the mixture of a salt and an acid or a base mixed in any particular quantities.

   Statement (II): Blood is a naturally occurring buffer solution whose pH is maintained by H₂CO₃/HCO₃^– concentrations.

   In the light of the above statements, choose the correct answer from the options given below:

   Answer: Statement II is correct, but Statement I is incorrect.
3. Q3: Solubility of calcium phosphate (molecular mass, M) in water is W g per 100 mL at 25°C. Its solubility product at 25°C will be approximately.
   \[ K_{sp} = [\text{Ca}^{2+}]^3[\text{PO}_4^{3-}]^2 \] Answer: Dependent on W.
4. Q4: Aqueous solution of ammonium carbonate is basic. The acidic/basic nature of a salt solution of a salt of weak acid and weak base depends on K_a and K_b values of the acid and base forming it.
   Answer: Both statements are correct.
5. Q5: On passing HCl(g) through a saturated solution of BaCl₂, at room temperature white turbidity appears. When HCl gas is passed through a saturated solution of NaCl, sodium chloride is precipitated due to the common ion effect.
   Answer: Both statements are correct.
6. Q6: K_a for CH₃COOH is 1.8 \times 10^{-5} and K_b for NH₄OH is 1.8 \times 10^{-5}. The pH of ammonium acetate solution will be ________.
   \[ \text{pH} = 7 + \frac{1}{2}(\text{p}K_a - \text{p}K_b) = 7 \] Answer: 7
7. Q7: The pH of an aqueous solution containing 1 M benzoic acid (pK_a=4.20) and 1 M sodium benzoate is 4.5. The volume of benzoic acid solution in 300 mL of this buffer solution is ________ mL.
   \[ \text{pH} = \text{p}K_a + \log\left(\frac{V_{\text{salt}}}{V_{\text{acid}}}\right) \] \[ 4.5 = 4.2 + \log\left(\frac{V_{\text{salt}}}{V_{\text{acid}}}\right) \] \[ \log\left(\frac{V_{\text{salt}}}{V_{\text{acid}}}\right) = 0.3 \] \[ \frac{V_{\text{salt}}}{V_{\text{acid}}} = 2 \implies V_{\text{acid}} = \frac{300}{3} = 100 \, \text{mL} \] Answer: 100 mL
8. Q8: For a sparingly soluble salt AB₂, the equilibrium concentrations of A²⁺ ions and B^– are 1.2 \times 10^{-4} M and 0.24 \times 10^{-3} M, respectively. The solubility product of AB₂ is:
   \[ K_{sp} = [\text{A}^{2+}][\text{B}^-]^2 = (1.2 \times 10^{-4})(0.24 \times 10^{-3})^2 \] \[ K_{sp} = 6.912 \times 10^{-11} \] Answer: 6.91 \times 10^{-11}
9. Q9: If the pK_a of lactic acid is 5, then the pH of 0.005 M calcium lactate solution at 25°C is 8.5 \times 10^{-1} (Nearest integer).
   Answer: 8.5
10. Q10: A litre of buffer solution contains 0.1 mole of each of NH₃ and NH₄Cl. On the addition of 0.02 mole of HCl by dissolving gaseous HCl, the pH of the solution is found to be 9.079 \times 10^{-3}.
    Answer: 9.079