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  4. 0.1 M of HA is titrated with 0.1 M NaOH, calculate the pH at end point. Given, Ka ( HA) = 5 × 10 - 6 α < < 1.

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Q. $0.1 \,M$ of $HA$ is titrated with $0.1 \,M \,NaOH$, calculate the pH at end point. Given, $ K_a \, ( HA) = 5 \times 10^{ - 6 } \, \alpha < < 1$.

IIT JEEIIT JEE 2004Equilibrium

Solution:

At the end-point, $ [A^- ] = 0.05$
$ K_b \, ( A^- ) = K_w / K_a = 2 \times 10^{ - 9 } $
$ [ OH^- ] = \sqrt{ K_b C } = \sqrt{ 2 \times 10^{ - 9 } \times 0.05 } = 10^{ - 5 } $
$pOH = 5$ and $pH = 9$