What is the pH value of a neutral solution?

The pH value of a neutral solution is 7.

A neutral solution is one in which the concentration of hydrogen ions $[H^{+}]$ is equal to the concentration of hydroxide ions $[O{H}^{-}]$. This equilibrium is expressed by the ion product of water:

$$K_w=[H^{+}][O{H}^{-}]=1.0\times 10^{-14}\text{(at 25°C)}$$

Since $[H^{+}]=[O{H}^{-}]$ in a neutral solution, we have:

$$[H^{+}]=\sqrt{K_w}=\sqrt{1.0\times 10^{-14}}=1.0\times 10^{-7}\text{mol/L}$$

The pH scale is defined as:

$$\text{pH}=-\log [H^{+}]$$

Substituting the value of $[H^{+}]$:

$$\text{pH}=-\log (1.0\times 10^{-7})=7$$

Thus, the pH of a neutral solution is 7. This value applies to pure water and any solution where $[H^{+}]=[O{H}^{-}]$.