If a solution has pH 2.5 then is it acidic or alkaline and what are the concentrations of H+ and OH- ions?

1 Answer
George C.
Nov 9, 2016

The solution is acidic with:

H+ concentration 10−2.5≈3.2×10−3 or 0.32%

OH- concentration 10−11.5≈3.2×10−12

Explanation:

In a neutral solution the concentration of H+ ions (actually H3O+) and OH- ions will both be about 10−7.

As the concentration of H+ ions increases, the concentration of OH- ions decreases in proportion and vice versa.

As a result, the product of the concentrations will always be about 10−14

We call solutions with more H+ than OH- acidic.

pH is minus the common (base 10) logarithm of the hydrogen ion concentration and pOH is minus the log of the hydroxyl ion concentration.

Hence pH + pOH = 14.

If our solution has pH 2.5, then that means that the concentration of H+ ions is 10−2.5≈3.2×10−3

pOH will be 14−2.5=11.5 and hence the concentration of OH- ions will be 10−11.5≈3.2×10−12

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