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Question: The pKa of a weak acid (HA) is 4.5. The pH of an aqueous buffered solution of HA, in which 50% of th...

The pKa of a weak acid (HA) is 4.5. The pH of an aqueous buffered solution of HA, in which 50% of the acid is ionized, is :

A

9.5

B

7.0

C

4.5

D

2.5

Answer

9.5

Explanation

Solution

For acidic buffer, pH = pKa + [A−][HA]\frac{\lbrack A^{-}\rbrack}{\lbrack HA\rbrack}

when the acid is 50% ionised, [A–] = [HA]

or pH = pKa + log 1 or pH = pKa

given pKa = 4.5

∴\therefore pH = 4.5 ∴\therefore pOH = 14 – 4.5 = 9.5.