Question

Is H2PO4- stronger than HCN?

Wouldn't HCN be stronger since H2PO4- predominantly acts as a base in aqueous solution?

Answer 1

This doesn't really matter. `H_2PO_4^-` has a greater `K_a` value than HCN, and is a stronger acid.

Explanation:

First of all, `H_2PO_4^-` is amphoteric - it can both donate and accept `H^+` ions.

The best indicator of acid strength is to look up the acid dissociation constant in a table like this one:

https://www2.chemistry.msu.edu/courses/cem262/aciddissconst.html

This number is an equilibrium constant that refers to the ability of the acid to donate `H^+` to water. It has the advantage of not being concentration dependent, so it removes any difficulty in having different concentrations when trying to compare acid strength.

Because an acid can be defined as a substance that can donate `H^+` ions to another substance, it makes sense to refer to a value like this that indicates just that ability.

For HCN, `K_a` is `6.2xx10^(-10)`,

For `H_2PO_4^-` you check `K_(a2)` of phosphoric acid `H_3PO_4`, as the ion is what exists after phosphoric acid dissociates the first time (hence second dissociation constant).

At any rate, the value is `6.32xx10^(-8)`, a larger value than for HCN, hence a stronger acid.