# What is the difference between the pH of a weak acid compared to the pH strong acid of the same concentration?

Jun 26, 2017

The $p H$ of the strong acid will be more negative at equilibrium.

#### Explanation:

We assess the strength of an acid on the basis of the completion of the protonolysis reaction as shown........

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

For strong acids, i.e. HX(X!=F); H_2SO_4;HClO_4;HNO_3 this equilibrium lies STRONGLY to the right as written.........and at equilibrium they are usually considered to be stoichiometric in ${H}_{3} {O}^{+}$ and ${X}^{-}$.

On the other hand, for weak acids, i.e. HX=HF;HO_2C-CH_3; HNO_2;H_3PO_4, the equilibrium tends to lie towards the right, and there are quantities of undissociated, free acid at equilibrium.

Now given that $p H = - {\log}_{10} \left[{H}_{3} {O}^{+}\right]$, the $p H$ of a STRONG acids should thus be more negative than the $p H$ of a WEAK acid of equivalent concentration.