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JEE Advanced levelSingle correct optionDifficulty 3 of 3pn110

One mole of pyrophosphoric acid, $H X_{4} P X_{2} O X_{7}$ , is (a) neutralised directly with NaOH using all replaceable hydrogens, and (b) first hydrolysed completely to $H X_{3} PO X_{4}$ and then neutralised with NaOH. The ratio of moles of NaOH in (a) : (b) is

A2 : 3
B3 : 2
C4 : 3
D1 : 1

Answer: A

(a)

H X_{4} P X_{2} O X_{7}

is tetrabasic, so 4 mol NaOH. (b)

H X_{4} P X_{2} O X_{7} + H X_{2} O 2 H X_{3} PO X_{4}

gives 2 mol of a tribasic acid, needing 6 mol NaOH. Ratio = 4 : 6 = 2 : 3.

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