Question

1. Without doing any calculations, choose the 0.12 M aqueous solutions with a pH of 2.

(Select all that apply.)

Mg(CH₃COO)₂

HNO₂

NH₄Cl

NaF

Ba(OH)₂

NaHCO₃

none of the above

2.

Without doing any calculations, choose the 0.12 M aqueous solutions with a pH between 2 and 6.

(Select all that apply.)

Mg(CH₃COO)₂

HNO₂

NH₄Cl

NaF

Ba(OH)₂

NaHCO₃

none of the above

Answer 1

Ans. #1. For getting a pH of 2.0 at 0.12M concertation, the acid must be a strong acid (like HCl, HNO₃, etc.) with very high dissociation constant.

# Option A. Incorrect. Mg(CH₃COO)₂ is a salt of weak acid (acetic acid) and strong base. When dissolved in water, it dissociates into Mg²⁺ and 2 CH₃COO^-. The acetate ion CH3COO^- acts conjugate base and accepts a proton from water as follow-

            CH₃COO^-(aq) + H₂O(l) -------> CH₃COOH(aq) + OH^-

Since the reaction forms OH^- ions, the pH of solution rises above 7.0 and becomes basic.

# Option B. Incorrect. HNO₂ is a weak acid with pKa of 3.14. So, the resultant solution shall have a pH above 3.4.

# Option C. Incorrect. NH₄Cl. NH4Cl is a salt of weak base (NH₃) and strong acid. The NH₄⁺ ions act as conjugate acid in water as follow-

            NH₄⁺ + H₂O -----> NH₃ + H₃O⁺

The solution is relatively acidic. However, since NH₄⁺ is a weak conjugate acid (pKa of 9.9), the pH shall be around 5.0

# Option D. Incorrect. NaF is a salt of weak acid (HF) and strong base.

The fluoride ion, acting as conjugate base, reacts with water to produce OH^- ion as follow-

            F^- + H₂O -------> HF + OH^-

So, the pH of solution is basic, above 7.0.

# Option E. Incorrect. Ba(OH)₂ is a strong base. The resultant solution shall have a pH much above 7.0.

# Option F. NaHCO₃. NaHCO₃ is a salt of weak acid and strong base. It dissociates to produce HCO₃^-. HCO₃^- has a pKa of 10.33, i.e. it is a weaker acid than water. Therefore, HCO₃^- acts as a weak base in water. So, the resultant solution has a pH above 7.0.

# Correct option. None of the above.

#2. Correct option B. HNO₂is a weak acid with pKa of 3.14. So, the resultant solution shall have a pH above 3.4.