There are 55.346 moles in 1 liter of pure water. This is calculated by dividing the density of water, which is 997.07 grams per liter, by the molecular mass of water, which is 18.02 grams per mole.
Molarity is found by dividing the number of moles of a substance present by the size of the sample. Most often, it is used to find the concentration of a solute in an aqueous solution. For example, a mole of sodium chloride dissolved in 1 liter of solution has a molarity of 1 mole per liter. Pure water is not a solution, so molarity is found by dividing grams per liter by grams per mole, giving an answer expressed in moles per liter.