Question

A buffer solution contains 0.3 M NH₄OH and 0.4 M NH₄Cl. If K_b for NH₄OH is 1.8 × 10^-5, calculate pH of the solution.

Answer 1

Given : [NH₄OH] = 0.3 M; 

[NH₄Cl] = 0.4 M 

K_b =1.8 × 10 ; 

pH = ? 

pK_b = -log₁₀K_b 

= -log₁₀1.8 × 10^-5

= - (\(\bar 5.2553\))

= 4.7447

pOH = pK_b + log₁₀\(\frac{[Salt]}{[Base]}\)

= 4.7447 + log₁₀\(\frac{0.4}{0.3}\)

= 4.7447 + log₁₀1.333

= 4.7447 + 0.1248 

= 4.8695 

∵ pH + pOH = 14 

∴ pH = 14 – pOH 

= 14 – 4.8695

= 9.1305

∴ . pH = 9.1305.