Sodium acetate $\left( CH _{3} COONa \right)=0.10\, M$;

Acetic acid $\left( CH _{3} COOH \right)=0.01\, M$;

$pH$ of acidic buffer solution is given by

$pH = pK _{ a }+\log \frac{[\text { Salt }]}{[\text { Acid }]}$

$=4.57+\log \left(\frac{0.1}{0.01}\right)$

$=5.57$