Is H2PO4- stronger than HCN?

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

1 Answer
Dwight
Feb 5, 2018

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.

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