pH of a buffer solution decreases by 0.02 units when 0.12g of acetic acid is added to 250 ml of a buffer solution of acetic acid and potassium acetate at 270C. The buffer capacity of the solution is ?

0.02 unit change of PH occurs when 0.12/60 mol of acid is added to 250 ml of buffer solution.Therefore, 0.02 unit change of PH  occurs when 4( 2x 10-3 )   moles of acid is added to 1000 ml of buffer solution .Therefore ,1unit change in PH occurs, when moles of acid added to 1000 ml of solution=........?=  4×2×10-32×10-2  =   0.4