Question

40 ml of 0.1 M ammonia solution is mixed with 20 ml of 0.1 M HCl. What is the pH of the mixture? ($p{K}_b$ of
ammonia solution is 4.74)

• A. 4.74
• B. 2.26
• C. 9.26
• D. 5

Answer 1

The correct option is C 9.26

From the question we can understand that the number of moles of the weak base (ammonia solution) is more than the number of moles of the HCl. The ($p{K}_b$ value of the weak base has been given as 4.74.

Based on the Henderson-Hasselbach equation, the pH of a buffer solution can be calculated using the formula,
pH = 14 - ($p{K}_b$ - log [B⁺]/ [BOH]

[B⁺] is the molar concentration of the salt or the anion formed. [BOH] is the molar concentration of the weak base. Since it is a buffer solution and the concentration of the salt (NH₄Cl) and the weak base (NH₃) is equal, the fraction will be 1.

So, pH = 14 - 4.74 - log 1.

pH is 9.26.