# 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?

Jun 13, 2018

Add some conc. $H C l \left(a q\right)$...?

#### Explanation:

You gots a weak acid, for which ${H}_{3} {O}^{+}$ depends on the equilibrium...

$H X \left(a q\right) + {H}_{2} O \left(l\right) r i g h t \le f t h a r p \infty n s {H}_{3} {O}^{+} + {X}^{-}$

And ${K}_{a} = \frac{\left[{H}_{3} {O}^{+}\right] \left[{X}^{-}\right]}{\left[H X \left(a q\right)\right]}$

And as always...$p H = - {\log}_{10} \left[{H}_{3} {O}^{+}\right]$...and so to REDUCE the $p H$ we would have to add a STRONG ACID … conc, $H C l$, approx. $10 \cdot m o l \cdot {L}^{-} 1$ is a good bet...this dissociates completely to give stoichiometric ${H}_{3} {O}^{+}$...and thus when it is added to solution (and we follow PRECISELY this order of addition of acid to water!), $p H$ should DECREASE markedly without grossly increasing solution volume...