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

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Answer 1 · 2016-11-09T13:40:26

The solution is acidic with:

H+ concentration #10^(-2.5) ~~ 3.2 xx 10^(-3)# or #0.32%#

OH- concentration #10^(-11.5) ~~ 3.2 xx 10^(-12)#

In a neutral solution the concentration of H+ ions (actually #H_3O^+#) 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 xx 10^-3#

pOH will be #14 - 2.5 = 11.5# and hence the concentration of OH- ions will be #10^(-11.5) ~~ 3.2 xx 10^-12#