Question

100 ml solution (I) of buffer containing 0.1(M) HA and 0.2 (M) A^–, is mixed with another solution (II) of 100 ml containing 0.2(M) HA and 0.3(M) A^– .

After mixing what is the pH of resulting solution ?

Given pK_a of HA = 5

• A. 5 – log 5/3
• B. 5 + log 5/3
• C. 5 + log 2/5
• D. 5 – log 5/2

Answer 1

Answer: (B)

5 + log 5/3

Answer 2

After mixing total moles of

A^– = 100 × 0.2 × 10^–3 + 100 × 0.3 × 10^–3

= 100 × 10^–3 × 0.5 moles

After mixing total moles of

HA = 100 × 0.1 × 10^–3 + 100 × 0.2 × 10^–3

= 100 × 0.3 × 10^–3 moles

After mixing resulting pH = 5 + log$\frac{5}{3}$