[slvcs]

Where do i start with this question I am kind of lost.


Given that the stepwise dissociation constants for phosphoric acid are:
Ka1 = 7.5×10-3; Ka2 = 6.2×10-8; Ka3 = 4.8×10-13

To prepare 1.20 L of a buffer solution having an ionic strength of 0.150 and a pH of 7.25 would require:

find (mass) of NaH2PO4(anhydrous) and find (mass) of Na2HPO4(anhydrous).


THANKS   

[AWK]

Use Henderson-Hasselbach equation to obtain ratio [H₂PO₄^- ]/[HPO_42- ]
An ionic strenth is equal to [H₂PO₄^- ] + 3[HPO_42- ].

Hence you have a set of equation of type
x/y=a
and
x+3y=0.15

This attempt is of course only (byt quite good) approximation

Use K2

[slvcs]

why An ionic strenth is equal to [H2PO4- ] + 3[HPO42- ].

1:3 ratio ?  is it given ?

[AWK]

I=1/2 sigma(c_iZ_i²)= 2 x c₁ x 1² + 2 x c₂ x 1² + c₂ x 2² = c₁ + 3 x c₂

[Borek]

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.

NaH₂PO₄ is dissociated to Na⁺ and H₂PO₄^- - in identical concentrations. Assume NaH₂PO₄ concentration is x - what will be its share in the ionic strength?

Do the same for Na₂HPO₄, assuming concentration is y - you should get 3y.

Thus total ionic strength in this solution is x + 3y.