by using Henderson-Hasselblach equation:
we can get [A-]/[HA]
when PH = Pka + ㏒[A-]/[HA]
PH = Pka + ㏒ [acetate] / [acetic acid]
So by substitution: PH = 4 & Pka = 4.76
4 = 4.76 + ㏒[acetate] / [acetic acid]
∴[acetate] / [acetic acid] = 0.174
∴[acetic acid] / [acetate] = 1 / 0.174 = 5.747
