Predict wheather the salt Na2HPO4 will form an acidic or basic solution on dissolving in water .


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The two possible reaction that HPO42- may undergo on addition to water are :

HPO4 2-  (aq) <-----> H+ (aq) + PO43-  (aq)

HPO4 2-  (aq) + H2O  <------> H2PO4- (aq) + OH- (aq)

depending onwhich of these has larger equilibrium contant , that ion will cause the solution to be acidic or basic .In case of PO43- , the Ka3 is 4.2 x 10-13  . We must calculate the values of Kb from the value of Ka for its conjugate acid H2PO42- .Use the relationship :

                       Ka x Kb = Kw

we want to know Kb for the base HPO42- , knowing the value of Ka for conjugate acid H2PO4-

Kb ( HPO42-) x Ka (H2PO4 - ) = Kw = 1.0 x 10-14

Because Ka for H2PO4- is 6.2 x 10-8 from table ,we calculate Kb for HPO42- to be 1.6x10-7 . This is 105 times larger than Ka for HPO4 2-  thus, 

this reaction HPO4 2-  (aq) + H2O  <------> H2PO4- (aq) + OH- (aq)

predominates over  HPO4 2-  (aq) <-----> H+ (aq) + PO43-  (aq)

and thus the solution is basic


Basic solution