100 ml 0.1 `M H_3PO_4` solution is being titrated with 0.1 M NaOH solution. The pH of the reaction mixture keeps increasing with addition of NaOH. The successive dissociation constant of `H_3PO_4 " are " 10^(-3) , 10 ^(-6) and 10^(-14) ` respectively.
How much volume of the given NaOH must be added such that a buffer `H_2PO_4^(-) // HPO_4^(-2) ` of maximum capacity is formed.

Options

A. 100 ml
B. 150 ml
C. 200 ml
D. 250 ml

Answer

B

Explanation

` {:(H_3PO_4 +, NaOH to, NaH_2PO_4, +H_2O ),(10 m " moles " , 10 m " moles " ,0,),(-,-, ,10 m " moles " ):}`
` {:(NaH_2PO_4 +, NaOH to , Na_2HPO_4+ H_2O ),(10 m " moles " , 5 m " moles " ," -" ),( 5 m " moles " , , ):}`
To get buffer with maximum buffer capacity half of the `NaH_2PO_4` should be consumed.
In 1st step, To get 10 m moles NaOH, volume should be 100 ml.
In 2nd, step , To get 5 m moles NaOH , volume should be 50 ml
Total vol = 150 ml