Question

how many moles of water are in a litre of water?

Answer 1

density of water = 1g/cm³ 
1L = 1000 cm³
mass of 1L water = density×volume = 1g/cm³ ×1000cm³ = 1000g

molar mass of water = 18g
number of moles in 1000g = $\frac{1000}{18}\ moles=\boxed{55.55\ mole}$

Answer 2

There are 55.55 moles per litre of water .