Molecular mass of water is constant i.e mass of one mole of water i.e 18 g Molar mass is the mass of a substance depending on how many moles of substance is taken into consideration. Ex. A mole of H2O: 18 g Two moles of H2O: 36 g Three moles of H2O: 54g and so on. Right? And molecular mass and molecular weight are same ,right?