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How to Convert Between Mass & Moles of a Substance

A pair is two, a dozen is 12 and a mole is 602,000,000,000,000,000,000,000. These words all express a distinct number of things. A water molecule is made up of two hydrogen atoms and one oxygen atom, but 2 grams of hydrogen and one gram of oxygen will not combine to make 3 grams of water. Oxygen atoms are heavier than hydrogen molecules, so a gram of oxygen contains fewer atoms than a gram of hydrogen. You need two moles of hydrogen and one mole of oxygen to make one mole of water molecules.

Instructions

- 1
  Weigh the substance. If the substance is a liquid or gas -- and for some solids -- this will mean weighing an empty container first and then weighing the container with the substance in it. Subtract the first weight from the second weight to get the weight of the substance.
- 2
  Find the moles per gram of substance you are measuring. If the substance is an element, this will be easy -- just look on the periodic table for atomic weight. If the substance is a compound, find the atomic weights of all the constitute elements and use the formula for the substance to compute the molecular weight. For example, the formula for sulphuric acid is H2SO4. There are two hydrogen atoms, one sulphur atom and four oxygen atoms in each sulphuric acid molecule. To find the molecular weight of a molecule of sulphuric acid, add two times the atomic weight of hydrogen, the atomic weight of sulphur and four times the atomic weight of oxygen.
- 3
  Divide the mass of the substance by the molecular weight of the substance to get the moles in your sample. If you know how many moles you want and the molecular mass, multiple the moles by the molecular weight to find out how many grams you need.

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