Explanation

(a)
HA (a weak acid) ionises as
$HA\ \ \ \ \ \ \ \ \ \ \ \ \rightleftharpoons H^{+}\ \ \ + {\ A}^{-}$
1 0 0
$\left( 1 - 10^{- 5} \right)\ \ \ \ \ \ \ \ 10^{- 5}\ \ \ \ \ \ \ 10^{- 5}\$
$\left\lbrack A^{-} \right\rbrack = \left\lbrack H^{+} \right\rbrack = 10^{- pH} = 10^{- 5}M$
$K_{a} = \frac{\left\lbrack H^{+} \right\rbrack\lbrack A^{-}\rbrack}{\lbrack HA\rbrack} = \frac{10^{- 5} \times 10^{- 5}}{(1 - 10^{- 5})} = \frac{10^{- 10}}{1}$ $(\therefore 1 > > > 10^{- 5})$
=$1 \times 10^{- 10}\ M$