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

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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##.