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QUESTION

# What is the Ka of a strong acid like HCl?

The approximate value of pKa is -7 for HCl.

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.