What is the weak ionization constant (K_a) for HCN equal to?

Sep 5, 2016

${K}_{a} \text{(hydrogen cyanide)} = {10}^{- 9.21}$

Explanation:

We assess the reaction:

H-C-=N + H_2O rightleftharpoons""^(-)C-=N + H_3O^+

And ${K}_{a}$ $=$ ${10}^{- 9.21}$ $=$ ([H_3O^+][""^(-)C-=N])/([HC-=N])

Note that the value of ${K}_{a}$ derives from careful and meticulous measurment under a standard set of conditions. The equilibrium is the reason why salts of hydrogen cyanide, i.e. $K C N$ or $N a C N$, give a slightly basic solution upon dissolution in water.