Equilibrium Question 263
Question: The dissociation constant of a weak acid is $ 1.0\times {10^{-5}}, $ the equilibrium constant for the reaction with strong base is [MP PMT 1990]
Options:
A) $ 1.0\times {10^{-5}} $
B) $ 1.0\times {10^{-9}} $
C) $ 1.0\times 10^{9} $
D) $ 1.0\times 10^{14} $
Show Answer
Answer:
Correct Answer: C
Solution:
HA ⇌ $ {H^{+}}+{A^{-}} $ ; $ K_{a}=\frac{[{H^{+}}][{A^{-}}]}{[HA]} $ –(i) neutralization of the weak acid with strong base is $ HA+O{H^{-}} $ ⇌ $ {A^{-}}+H_2O $
$ K=\frac{[{A^{-}}]}{[HA][O{H^{-}}]} $ –(ii) dividing (i) by (ii) $ \frac{K_{a}}{K}=[{H^{+}}][O{H^{-}}] $
$ =K_{w}={10^{-14}} $
$ K=\frac{K_{a}}{K_{w}}=\frac{{10^{-5}}}{{10^{-14}}}=10^{9} $ .
