# pH of a buffer solution decreases by $0.02$ units when $0.12$ g of acetic acid is added to $250\;ml$ of a buffer solution of acetic acid and Potassium acetate at $27^{\circ}$. The buffer capacity of the solution is :

$\begin {array} {1 1} (1)\;0.1 & \quad (2)\;10 \\ (3)\;1 & \quad (4)\;0.4 \end {array}$

(4) 0.4