TS EAMCET 6-Aug-2021 Shift 1 Question Paper

For a tribasic acid like H3PO4 has three ionisation constant. The overall ionisation constants (K) will be product of ionisation constants of three steps, i.e.
K=K1×K2×K3
H3PO4⇌H++H2PO4−
K1=‌

[H+][H2PO4−]

[H3PO4]

. . . (i)
H2PO4−⇌H++HPO42−
K2=‌

[H+][HPO42−]

[H2PO4−]

. . . (ii)
HPO42−⇌H++PO43−
K3=‌

[H+][PO43−]

[HPO42−]

. . . (iii)
Adding Eqs. (i), (ii) and (iii), we get
H3PO4‌⇌3H++PO43−
K‌‌=‌

[PO43−][H+]3

[H3PO4]

Multiply mathematical Eqs. (i), (ii) and (iii), we get
K1×K2×K3=‌

[H+][H2PO4−]

[H3PO4]

×‌

[H+][HPO42−]

[H2PO4−]

×‌

[H+][PO43−]

[HPO42−]

K1K2K3=‌

[H+]3[PO43−]

[H3PO4]

Overall ionisation constant K,K=K1×K2×K3