Calculate pH of a 0.1 M `Na_2 HPO_4` solution. `K_1 , K_2` and `K_3` for `H_3PO_4` are `7.1 xx 10^(-3), 6.3 xx 10^(-6) ` and `4.5 xx 10^(-13)` respectively. Which approximation is necessary for the calculation?

Explanation

As `K_3` for `H_3PO_4` is very low compared to `K_1` assume no acidic ionisation. Consider only the equilibrium,
`HPO_4^(2-) + H_2O iff H_2PO_4^(-) + OH^-`
(10,10)