The number of ${\mathrm{H}}_3{\mathrm{O}}^{+}$ ions present in 10ml of water at ${25}^{°}\mathrm{C}$ is

At ${25}^{°}\mathrm{C} {\mathrm{H}}_3{\mathrm{O}}^{+}={10}^{-7}\mathrm{M} 1000\mathrm{mL}$ water contains ${10}^{-7}$ moles of ${\mathrm{H}}_3{\mathrm{O}}^{+}$

10 ml water contains$=\frac{{10}^{-7}\times {10}^1}{{10}^3}={10}^{-9}$

1 mole contains $6.023\times {10}^{23}$ ions 

Total number of ${\mathrm{H}}_3{\mathrm{O}}^{+}$ ions$=6.023\times {10}^{23}\times {10}^{-9}=6.023\times {10}^{14}$