Question

A buffer contains equal amounts of a weak acid and its conjugate base and has a pH of 5.25.

Which would be a reasonable value of buffer pH after the addition of a small amount of acid?
A) 6.35
B) 5.15
C) 5.35
D) 4.15

Answer 1

option B 5.15

Step-by-step explanation:

`weak\ acid \rightleftharpoons conjugate\ base+H^+`

Henderson–Hasselbalch equation for pH is as follows:

`pH=pKa+log([conjugate\ base])/([weak\ acid])`

Given:

[conjugate base] = [weak acid]

pH = 5.25

`pH=pKa+log([conjugate\ base])/([weak\ acid])`

5.25 = pKa+log 1

pKa = 5.25

When acid is added, H+ concentration increases and as per Le Chatelier's principle, equilibrium shifts towards left hand side or concentration of weak acid increases and that of conjugate base decreases.

So,
`([conjugate\ base])/(weak\ acid)`<1

So,
`log([conjugate\ base])/([weak\ acid]) =small\ negative\ value`

Therefore, pH of the solution will be slightly less than the initial.

Therefore, pH of the solution will be 5.15.

So, the correct option is option B