why An ionic strenth is equal to [H2PO4- ] + 3[HPO42- ].
1:3 ratio ? is it given ?
Check what is ionic strength definition, formula given by AWK simply follows.
NaH
2PO
4 is dissociated to Na
+ and H
2PO
4- - in identical concentrations. Assume NaH
2PO
4 concentration is x - what will be its share in the ionic strength?
Do the same for Na
2HPO
4, assuming concentration is y - you should get 3y.
Thus total ionic strength in this solution is x + 3y.