Question

HCN has a pKa of 9.21. You have to use CN- for a reaction, but do not want to create any toxic gaseous HCN. What is the minimum pH at which you should perform the reaction so that you don’t harm yourself? Explain your reasoning.

Answer 1

Ans. Being weak acid, HCN dissociates to yield CN^- and H⁺ in aqueous solution as follow-

            HCN + H₂O <-------> CN^- + H₃O⁺

# Since the dissociation reaches an equilibrium, there must be all the chemical species at equilibrium.

# Let’s assumed that when, 99.99% of original [HCN] is in dissociated form, the remaining [HCN] is below safe level for working condition.

So, on a 100-unit scale (arbitrary unit of concentration)

            Initial [HCN] = 100

            Equilibrium [HCN] = Initial [HCN] – 99.99% of initial [HCN]

                                                = 100 – (99.99 % x 100)

                                                = 0.01

            Equilibrium [CN^-] = 99.99 % of Initial [HCN] = 99.99

Let the required [H₃O⁺] be X.

Ka of HCN = 10^-pKa = 10^-9.21 = 6.166 x 10^-10

Now,

            Ka = [CN^-] [H₃O⁺] / [HCN]               - all conc. At equilibrium

            Or, 6.166 x 10^-10 = (99.99 x X) / 0.01 = 9999X

            Or, X = 6.166 x 10^-10 / 9999 = 6.6166 x 10^-14

# Therefore, equilibrium [H₃O⁺] = 6.6166 x 10^-14

Now,

            pH = -log [H₃O⁺] = -log 6.6166 x 10^-14 = 13.21

Therefore, required pH = 13.21

Note: You can adjust the % permitted dissociation for safety level as needed.