Answer
55.51 mol
$3.342\times10^{25}molecules$
Work Step by Step
Assume that the water’s density is $1000\frac{kg}{m^3}$.
$$1.000L\frac{10^{-3}m^3}{1L}\frac{1000kg}{1m^3}\frac{1mol}{(15.999+2(1.008))\times10^{-3}kg}=55.51mol$$
$$55.51mol\frac{6.02\times10^{23}molecules}{mole}=3.342\times10^{25}molecules$$