Question

What alterations would have to be made for a solution of a weak acid to have a lower pH than a solution of a strong acid?

Answer 1

Add some conc. `HCl(aq)`...?

Explanation:

You gots a weak acid, for which `H_3O^+` depends on the equilibrium...

`HX(aq) +H_2O(l) rightleftharpoonsH_3O^+ + X^-`

And `K_a=([H_3O^+][X^-])/([HX(aq)])`

And as always...`pH=-log_10[H_3O^+]`...and so to REDUCE the `pH` we would have to add a STRONG ACID … conc, `HCl`, approx. `10*mol*L^-1` is a good bet...this dissociates completely to give stoichiometric `H_3O^+`...and thus when it is added to solution (and we follow PRECISELY this order of addition of acid to water!), `pH` should DECREASE markedly without grossly increasing solution volume...