First slide

PH of strong acids and strong bases

Question

The $P^{H}$ of 0.1 M monobasic acid is 4.5. The $K_{a} and P^{K_{a}}$ values, respectively, are

Moderate

A

$1.0 \times 10^{- 6}, 6$
B

$1.0 \times 10^{- 7}, 7$
C

$1.0 \times 10^{- 8}, 8$
D

none of these

Solution

$P^{H} = - \log [H^{+}]$
$[H^{+}] = Antilog (- P^{H})$
$= Antilog (- 4.5)$
$= Antilog (\bar{5} . 5) = 3.16 \times 10^{- 5}$
$[H^{+}] = [A^{-}] = 3.16 \times 10^{- 5}$
$[HA] = C - Cα = C - [H^{+}]$
$= 0.1 - 3.16 \times 10^{- 5} = 0.1$
$K_{a} = \frac{[H^{+}] [A^{-}]}{[HA]} = \frac{{(3.16 \times 10^{- 5})}^{2}}{0.1} = 1.0 \times 10^{- 8}$
$P^{K_{a}} = - \log K_{a} = - \log 10^{- 8} = + 8$

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