A buffer solution 0.04 M in `Na_2HPO_4 and 0.02 ` M in `Na_3 PO_4` is prepared. The electrolytic oxidation of 1.0 milli -mole of the organic compound RNHOH is carried out in 100 mL of the buffer. The reaction is `RNHOH +H_2O to RNO_2 +4H ^(+) + 4e ^(-) `
The approximate pH of solution after the oxidation is complete is:
[Given : for `H_3O PO_4, pK_(a_1) =7.20 , pK_(a_2) =12]`

Options

A. ` 6.90 `
B. ` 7.20 `
C. ` 7. 5`
D. None of these

Answer

C

Explanation

` {:(PO_4^(3-) +,H^(+) to, HPO_4^(-2) ),( 0.02M , 0.04 M , 0.04 M),( -, 0.02 , 0.06M), (- , 0.02M, -):}`
` {:( HPO_4^(-2) ,H^(+) to, H_2PO_4^(-)),( 0.06, 0.02 , 0),( 0.02 , - , 0.02M),( 0.04M, , ):}`
` pH =7.2 +log ""(0.04)/(0.02 ) =7.5`