Question

Question: Why is the pH of water at 25^oC equal to 7.00 ?

Short answer

Answer

The pH scale represents the amount of ${\mathrm{H}}^{+}$ions present in the solution. Still, as it is a negative logarithm of ${\mathrm{H}}^{+}$ion concentration, the higher the value of ${\mathrm{H}}^{+}$ ions, the lower the value of pH.

At the concentration of ${\mathrm{H}}^{+}$ions in water, pH (i.e.,$-\log {\mathrm{H}}^{+}$) is 7.

Step-by-step solution

The pH of the solution

The pH measures the number of hydrogen ions, i.e., H⁺ ions present in a solution. The following formula can calculate it:

$\mathbf{pH}\mathbf{}\mathbf{=}\mathbf{-}\mathbf{log}\left[H^{+}\right]$

As the sign is negative, the higher the value of ions, the lower the pH of the solution.

The pH of water at  25oC. 

Experiment shows that at 25°C

$$\begin{gathered} \left[{\mathrm{H}}^{+}\right]=\left[{\mathrm{OH}}^{-}\right] \\ =1.0\times {10}^{-7}\mathrm{M} \\ \mathrm{pH}=-\log \left[{\mathrm{H}}^{+}\right] \\ =-\left(-7\right) \\ =7 \end{gathered}$$

Hence, at ${25}^{\mathrm{o}}\mathrm{C}\mathrm{pH}$of water is 7.