Question

If pkb is 9.7 calculate pH of 0.1 M NH4Cl solution

• A. 2.65
• B. 8.65
• C. 5.15
• D. 8.15

Answer 1

Answer: (A)

2.65

Answer 2

NH4Cl is a salt of a weak base (NH3) and a strong acid (HCl). The NH4+ ion hydrolyzes, making the solution acidic. The pH can be calculated using the formula for the hydrolysis of a salt of a weak base and strong acid: $pH = 7 - \frac{1}{2}(pK_b + \log C)$ Given $pK_b = 9.7$ and $C = 0.1$ M: $pH = 7 - \frac{1}{2}(9.7 + \log(0.1))$ $pH = 7 - \frac{1}{2}(9.7 - 1)$ $pH = 7 - \frac{1}{2}(8.7)$ $pH = 7 - 4.35$ $pH = 2.65$