The first and second dissociation constants of an acid H2A are 1.0×10−5 and 5.0×10−10 respectively. The overall dissociation constant of the acid will be:
A
5.0×1015
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B
5.0×10−15
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C
0.2×105
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D
5.0×10−5
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Solution
The correct option is B5.0×10−15 H2A⟷HA−+H+
K1=[HA−][H+][H2A]
HA−⟷H++A2−
K2=[A2−][H+][HA−]
for the reaction, H2A⟷A2−+2H+
K=[A2−][H+]2[H2A]
K=(K1)(K2)
=1×10−5×5×10−10
=5×10−15
overall dissociation constant of acid will be 5×10−15.