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

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

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Answer 1 · 2016-06-27T09:18:39

Possibly, but we don't really know.

Explanation:

For the protonolysis of an acid in water we write:

$H A (a q) + H_{2} O (l) ⇌ H_{3} O^{+} + A^{-}$ $HA(aq) + H_2O(l) rightleftharpoons H_3O^+ + A^-$

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 $0.0001$ $0.0001$ $H N O_{3} (a q)$ $HNO_3(aq)$ ; in water, $[H_{3} O^{+}] = [N O_{3}^{-}] = 10^{- 4} \cdot m o l \cdot L^{- 1}$ $[H_3O^+]=[NO_3^-]=10^-4*mol*L^-1$ , but $p H = 4$ $pH=4$ by definition. ( $p H = - {log}_{10} [H_{3} O^{+}]$ $pH=-log_10[H_3O^+]$ )

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

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Explanation:

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