bantu pake caranya :)

jawabannya yang (d).

pH penyangga
OH- = kb x cb/cg = 2x10^-5 x 0.2/0.4 = 10^-5
pOH = 5
pH = 14-5 = 9

tambah basa biar jadi 10 (berubah 1 satuan).
pH = 10
pOH = 4 => OH- = 10^-4

10^-4 = 2x10^-5 (0.2+x)/(0.4-x)
(0.4-x) = 2x10^-1 (0.2+x)
0.4-x = 0.04+0.2x
1.2 x = 0.36
x = 0.3 mol