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

1 Answer
anor277
Jun 27, 2016

Possibly, but we don't really know.

Explanation:

For the protonolysis of an acid in water we write:

#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# #HNO_3(aq)#; in water, #[H_3O^+]=[NO_3^-]=10^-4*mol*L^-1#, but #pH=4# by definition. (#pH=-log_10[H_3O^+]#)

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