How many water molecules are in a glass of water? This glass is filled with 200 ml water. That is 0,2 l of water. First, to get started, we need to convert volume to mass. Since 1l water equals 1kg water, then our 200 ml glass is filled with 0.2 kg water. In school chemistry probably in 8th grade there was a term Mole. Mole is a unit and it measures number of molecules. One mole has 6.0221415×10 23 molecules in it. And another term is molar mass. The molar mass of an element is the atomic weight in grams per mole. In other words molar mass states - how much does one mole of particular substance weight. The molar mass for distilled water is 18 g/mol or converting to kg, it is 0,018 kg / mol. So, now lets do some calculations. First, let's calculate - how many moles of water there are in the glass. Moles = mass / molar mass = 0,2 kg / (0,018 kg/mol) = 11.11 moles And then, let's calculate how many molecules there are in the 11.11 moles Number of molecules = number of mols * m
