# A solution of an acid in water has a pH of 4, which is slightly acidic. Is this solution a weak acid?

Jun 27, 2016

Possibly, but we don't really know.

#### Explanation:

For the protonolysis of an acid in water we write:

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

Now for a strong acid, the equilibrium lies strongly to the right; whereas for a weak acid, the equilbrium lies to the left. However, what if I take a dilute solution of a strong acid?

Suppose I have prepared $0.0001$ $H N {O}_{3} \left(a q\right)$; in water, $\left[{H}_{3} {O}^{+}\right] = \left[N {O}_{3}^{-}\right] = {10}^{-} 4 \cdot m o l \cdot {L}^{-} 1$, but $p H = 4$ by definition. ($p H = - {\log}_{10} \left[{H}_{3} {O}^{+}\right]$)