# Write the balanced chemical equation for the reaction of the weak acid HCN with water. Include the phase of each species. How do you complete the Ka expression for this reaction?

HC-=N(aq) + H_2O(l) rightleftharpoons H_3O^+ + ""^(-)C-=N(aq)
$p {K}_{a}$ $=$ $9.21$
K_a=([H_3O^+][""^(-)C-=N])/([HC-=N])=10^(-9.21)
Hydrogen cyanide was used as a war gas, and, criminally, as a means to execute people, both in concentration camps (as Cyclon B), and US prisons. Interestingly, looking around the web, the values of ${K}_{a}$ were inconsistent. The one I quoted was from this site. You would probably do better to quote the value given in your text.