Because chemist want to know the number of atoms or the number of moles but we can not count the number of atoms, we use the molar mass to convert the mass of the substance into moles or the moles into mass. If we use the molar mass as a unit factor, then we can do one of two calculation Given the mass of the substance: Mass / molar mass = moles For example how many moles are in 10.0 gram of water, H2O? The molar mass of water is 18.01 g/mol; therefore, 10.0 g / 18.01 g/mol = 0.555 moles. Given the moles of the substance: Moles x molar mass = mass For example, What is the mass of 5.00 moles of water? 5.00 moles x 18.01 g/mol = 90.1 g Calculate the number of MOLES in 7.57 grams of platinum, Pt. Group of answer choices 0.0388 moles 1477 moles 4.56 x 1023 moles 2.45 x 10 -21 mole