An organic nitro-compound (RNO2) is electrolytically reduced in an aqueous acetate buffer solution h — Organic Chemistry Chemistry Question
Reactions in buffer medium
An organic nitro-compound (RNO2) is electrolytically reduced in an aqueous acetate buffer solution having total acetate concentration (HOAc + OAc – ) 0.500 and pH = 5.0. The buffered solution with the volume of 300 cm 3 in which the concentration of RNO2 was 0.01 mol dm -3 , was reduced completely. The dissociation constant for acetic acid is 1.75×10 -5 at 25 °C.
The reduction reaction is
RNO2 + 4 H + + 4 e– → RNHOH + H2O
Calculate the pH of the solution after completion of the reduction of RNO2.
Model Answer
RNO2 + 4 H+ + 4 e- → RNHOH + H2O
HOAc = H+ + OAc-
[H+][OAc-] / [HOAc] = Ka
i. e.
pKa = pH + log ([HOAc] / [OAc-])
4.76 = 5.0 + log ([HOAc] / [OAc-])
[HOAc] / [OAc-] = 0.5715
[HOAc] + [OAc-] = 0.500
[OAc-] = 0.3182
[HOAc] = 0.5 - 0.3182 = 0.1818
mmoles of acetate (OAc-) present initially in 300 cm3 = 0.3182 × 300 = 95.45
mmoles of acetic acid (HOAc) present initially in 300 cm3 = 0.1818 × 300 = 54.55
mmoles of RNO2 reduced = 300 × 0.0100 = 3
From the stoichiometry of the equation:
3 mmoles of RNO2 will consume 12 moles of H+ for the reduction. The H+ is obtained from dissociation of HOAc.
On complete electrolytic reduction of RNO2,
mmoles of HOAc = 54.55 - 12.00 = 42.55
mmoles of OAc- = 95.45 + 12.00 = 107.45
4.76 = pH + log(42.55 / 107.45)
pH = 5.16