Question

How to calculate pKa of `HCl`?

Answer 1

The ionisation of HCl can be shown as:

`HCl + H_2O -> H_3O^+ + Cl^-`

`K_a` can be calculated as the ratio of the product of the concentrations of the products to that of the reactant, that is,

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

Since `HCl` is a strong acid, the value of `K_a` turns out to be very large, that is,

`K_a=10^7/1=10^7` (approx.)

The value of `pK_a` is given by

`pK_a=-logK_a`
`pK_a=-(log10^7)`
`pK_a=-(7)=-7`

You can learn more about finding the values of `K_a` and `pK_a` here.