Recall that HCN is hydrocyanic acid and is a weak acid. The Ka reaction would be an equilbrium reaction with water.

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Acid: H+ donorBase: H+ aceptor

HCN(aq) + H2O(l) CN–(aq) + H3O+(aq)(weak acid) (base) (conjugate base) (conjugate acid)

The Ka reaction of HCN is HCN(aq) + H2O(l) ⇌ CN–(aq) + H3O+(aq).

The Ka expression is:


For the next part, we are being asked to find Kb for CN–. CN– is the conjugate base of the weak acid HCN.

Hence, the Kb of CN– can be calculated from the Ka of HCN. Ka and Kbare connected by the autoionization constant of water (Kw)in the following equation:

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Problem Details

a. What is the K a reaction of HCN? The Ka of HCN is 6.2 × 10-10.

b. What is Kb value for CN– at 25°C?

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Based on our data, we think this problem is relevant for Professor Golde's class at PITT.

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