Equal volumes of three strong acid solutions of pH 3, 4 and 5 are mixed in a vessel. What will be the ${\mathrm{H}}^{+}$ion concentration in the mixture?

Molarity of the mixture 

$\mathrm{M}=\frac{{\mathrm{M}}_1{\mathrm{V}}_1+{\mathrm{M}}_2{\mathrm{V}}_2+{\mathrm{M}}_2{\mathrm{V}}_3}{\mathrm{V}}$

For 1st solution $(\mathrm{pH}=3)\left[{\mathrm{H}}_3{\mathrm{O}}^{+}\right]={10}^{-3}\mathrm{M}$

For 2nd solution $(\mathrm{pH}=4)\left[{\mathrm{H}}_3{\mathrm{O}}^{+}\right]={10}^{-4}\mathrm{M}$

For 3rd solution $(\mathrm{pH}=5)\left[{\mathrm{H}}_3{\mathrm{O}}^{+}\right]={10}^{-5}\mathrm{M}$

$$\begin{gathered} \left[{\mathrm{H}}^{+}\right]=\frac{V\left[{10}^{-3}+{10}^{-4}+{10}^{-5}\right]}{3\mathrm{V}} \\ =3.7\times {10}^{-4} \end{gathered}$$