What is the Ka of a strong acid like HCl?

1 Answer
Dr. Hayek
Aug 23, 2016

The approximate value of #pKa# is #-7# for #HCl#.

Explanation:

Hydrochloric acid is a strong acid and it is considered to completely dissociate in water under the following equation:

#HCl(aq)+H_2O(l)->H_3O^(+)(aq)+Cl^(-)(aq)#

The equilibrium constant #K_a# is written as:

#K_a=([H_3O^+][Cl^-])/(HCl]#

Since the acid dissociates completely, the concentration of products (#[H_3O^+] and [Cl^-]#) is very large and the concentration of the reactant (#[HCl]#) is very small. Therefore, the value of #K_a# is going to be very large (#~10^(7)#).

Since #pK_a=-logK_a#, therefore, the value of the #pK_a# is going to be a negative number.

The approximate value of #pKa# is #-7# for #HCl#.

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