, 
and 
values for H 3 PO 4 are 10 –3 , 10 –8 and 10 –12 respectively. If K w (H 2 O) = 10 –14 , then :
(i) What is dissociation constant of HPO 4 2– ?
(ii) What is K b of HPO 4 2– ? (iii)
What is K b of H 2 PO 4 – ?
(iv) What is order of K b of PO 4 3– ( 
), HPO 4 2– ( 
) and H 2 PO 4 – ( 
) ?
Text Solution
Verified by ExpertsCHECK THE SOLUTION.
(i) 10 –12 (ii) 10 –6
(iii) 10 –11 (iv) 
< 
< 
Sol. (i) HPO 4 2– 
H + + PO 4 3– ; K eq = 
of H 3 PO 4 = 10 –12
(ii) K a (H 2 PO 4 – ) × K b (HPO 4 2– ) = K w = 10 –14 . So, 10 –8 × K b (HPO 4 2– ) = 10 –14 . Therefore, K b (HPO 4 2– ) = 10 –6 .
(iii) K a (H 3 PO 4 ) × K b (H 2 PO 4 – ) = K w = 10 –14 . So, 10 –3 × K b (H 2 PO 4 – ) = 10 –14 . Therefore, K b (H 2 PO 4 – ) = 10 –11 .
(iv) K b of PO 4 3– ( 
) = 
, K b of HPO 4 2– ( 
) = 
and K b of H 2 PO 4 – ( 
) = 
.
Since order of K a is 
> 
> 
, so order of K b is 
< 
< 
.
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