Question: how many molecules are in 1 kg of water?
Answer: Take the reciprocal of the molar mass (1/18.01528 mol/g) and multiply by 1000g (equivalent of 1kg). This will give you the number of moles in 1 liter of water. Finally, multiply this number of moles by Avogadro's number (6.022 E23 molecules/mol) to find the number of water molecules in the liter of water.