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

Nov 9, 2016

The solution is acidic with:

H+ concentration ${10}^{- 2.5} \approx 3.2 \times {10}^{- 3}$ or 0.32%

OH- concentration ${10}^{- 11.5} \approx 3.2 \times {10}^{- 12}$

#### Explanation:

In a neutral solution the concentration of H+ ions (actually ${H}_{3} {O}^{+}$) 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 ${10}^{- 2.5} \approx 3.2 \times {10}^{-} 3$

pOH will be $14 - 2.5 = 11.5$ and hence the concentration of OH- ions will be ${10}^{- 11.5} \approx 3.2 \times {10}^{-} 12$