How to Calculate the Volume of Molarity
- 1). Suppose you know the mass of the substance for which you are trying to find the molar volume. Determine the atomic weight of the substance by looking up the atomic weight of the substance's constituent elements. For example, if the only substance is gaseous water vapor, then look up the weight in atomic mass units (amu) for hydrogen and oxygen. Many periodic tables of the elements will list the amu under each element symbol. For hydrogen and oxygen, they are 1.0079 amu and 15.9994 amu. So a molecule of water weighs 2(1.0079)+15.9994 = 18.015 amu.
- 2). Convert the amu into grams. An amu is define as one twelfth the mass of a carbon-12 molecule. A mole is defined so that 12 grams of carbon-12 contains a mole of molecules. So 12 amu/molecule corresponds to 12 g/mol. So 18.015 amu/molecule of water vapor corresponds to 18.015 g/mol.
- 3). Suppose the known mass of water vapor is 0.500 g in a container of 5 liters (L). Then solve for the number of moles by dividing the mass by the density to get 0.500/18.015 = 0.0278 moles. Notice that only three significant figures were retained because the mass was known to only three significant figures.
- 4). Divide the mole count by the volume to get the molarity. 0.0278/5=0.00556 mol/L.
- 5). Take the reciprocal to get the molar volume, or volume per mole. 1/0.00556 mol/L = 180. L/mol. Including the decimal point conveys that the 0 is a significant figure, too.