Question

Which of the statements are incorrect?

• A. pH of a solution of salt of stong acid and weak base is less than 7.
• B. pH of a solution of a weak acid and weak base is kb<ka'
• C. pH of an aqueous solution of 10-8 (M) HCI is 8.
• D. Con jugate acid of NH2- is NH3-

Answer 1

The Correct Option is B, C

• pH of a solution of a salt of a strong acid and weak base is less than 7: This statement is correct. A strong acid/weak base salt will hydrolyze in water, producing H⁺ ions and lowering the pH below 7.
• pH of a solution of a weak acid and weak base is $k_b < k_a$: This statement is incorrect. The pH of a weak acid/weak base depends on the relative strengths of the acid (K_a) and the base (K_b).
  • If K_a > K_b, the solution is acidic (pH < 7).
  • If K_a < K_b, the solution is basic (pH > 7).
  • If K_a = K_b, the solution is approximately neutral (pH ≈ 7).
• pH of an aqueous solution of 10^-8 M HCl is 8: This statement is incorrect. HCl is a strong acid, so it will decrease the pH of water. Even though the concentration is very low, the pH *must* be less than 7. You cannot have an acidic solution with a pH of 8. The pH will be slightly less than 7, taking into account the autoionization of water.
• Conjugate acid of NH₂^- is NH₃^-: This statement is incorrect. The conjugate acid of NH₂^- is formed by adding a proton (H⁺). Thus, NH₂^- + H⁺ → NH₃. The correct conjugate acid is NH₃, not NH₃^-. NH₃ is the conjugate acid, which will be uncharged.

Correct Answer: pH of a solution of a weak acid and weak base is $k_b < k_a$, pH of an aqueous solution of 10^-8 M HCl is 8, Conjugate acid of NH₂^- is NH₃^-