Ionic Equilibrum

20 mL of 0.1 M acetic acid is mixed with 50 mL of potassium acetate. $ K_{\alpha} $ of acetic acid $ =1.8 \times 10^{-5} $ at $ 27^{\circ} \mathrm{C} $. Calculate the concentration of potassium acetate if pH of mixture is 4.8 .