pH of an acetate buffer after dilution
Depending on your first calculations before dilution:
\begin{align}
[\ce{CH3COO-}]&amp;=[\ce{CH3COONa}]=\pu{6.4\times{10^{-5} M}}=[\ce{Na+}]\\ 
[\ce{Na+}]_\text{(After dilution)}&amp;=\pu{6.4\times{10^{-6} M}}\\ 
[\ce{CH3COO-}]+[\ce{CH3COOH}]&amp;=\pu{1\times{10^{-4} M}}~\text{ (Before dilution)}\\
[\ce{CH3COO-}]+[\ce{CH3COOH}]&amp;=\pu{1\times{10^{-5} M}}~\text {(After dilution)}
\end{align}
So,you have the the following $5$ independent equations in order to specify the unknown concentrations of $5$ species as present in an aqueous solution consisting of $\pu{1\times{10^{-5} M}}$  buffer solution made from acetic acid and sodium acetate :

\begin{align}
6.4 \times 10^{−6} &amp;= [\ce{Na+}] \tag{1}\\
K_\mathrm{a} &amp;= \frac{[\ce{H+}][\ce{CH3COO-}]}{[\ce{CH3COOH}]} 
     = 1 \times 10^{−4.75} \tag{2}\\
K_\mathrm{w} 
    &amp;= [\ce{H+}][\ce{OH-}] 
     = 1 \times 10^{−14} \tag{3}\\
1\times{10^{-5}}
    &amp;=[\ce{CH3COOH}] + [\ce{CH3COO-}]\tag{4}\\
[\ce{H+}]+[\ce{Na+}] 
    &amp;= [\ce{OH-}] + [\ce{CH3COO-}] \tag{5}
\end{align}