Physics + Chemistry

First lets convert the volume in to cubic meter.So volume of cylinder is $\frac{2500}{100^3}$ = $\frac{1}{400}$ $m^3$ .As the water is pure so the density should be $1000$ $kg$ $m^{-3}$ .Threrefore the mass of water in cylinder is $\frac{1}{400}$ * $1000$ = $2.5$ $kg$ .Now $6.023$ * $10^{23}$ molecules are present in $18$ $g$ = $18$ * $10^{-3}$ $kg$ water.So number of molecules is { $2.5$ * ( $6.023$ * $10^{23}$ )} ÷ ( $18$ * $10^{-3}$ )= $8.365$ * $10^{25}$