To neutralize completely 20 mL of 0.1 M aqueous solution of phosphorus acid, the volume of 0.1 M aqueous NaOH solution required is

Step-by-step solution

H 3 P O 3 is a dibasic acid (containing two ionisable protons attached to O directly) H 3 P O 3 ↔ 2 H + +HP O 3 2 - 0 .1 M H 3 PO 3 = 0 .2 N H 3 PO 3 0 .1 M NaOH = 0 .1 N NaOH N 1 V 1 = N 2 V 2 NaOH H 3 P O 3 0.1 × V 1 = 0.2 × 20 V 1 = 40 mL Alternative short method H 3 P O 3 + 2 N a O H → N a 2 H P O 3 + 2 H 2 O M A V A x A = M B V B x B …(i) Here x A and x B are balancing coefficient of acid and base respectively. Now putting all the values in equation (i) we get 20 × 0.1 1 = 0.1 × V B 2 V B = 40   ml