Question

In pure HCOOH liquid, concentration of $HCO{O}^{-}={10}^{-3}M$ at ${27}^{0}C$. What is the self ionisation constant at ${27}^{0}C$? $(k=[HCOO{H}_2^{+}\left]\right[HCO{O}^{-}\left]\right)$ ?

• A. ${10}^{-3}$
• B. ${10}^3$
• C. ${10}^6$
• D. ${10}^{-6}$

Answer 1

Answer: (D)

${10}^{-6}$

$2HCOOH\to HCOO{H}_2^{+}+HCO{O}^{-}$

At equilibrium the concentration of $HCOO{H}_2^{+}$ and $HCO{O}^{-}$ are the same. So, $\left[HCOO{H}_2^{+}\right]=\left[HCO{O}^{-}\right]={10}^{-3}M$

$k=\left[HCOO{H}_2^{+}\right]\left[HCO{O}^{-}\right]={10}^{-3}\times {10}^{-3}={10}^{-6}$