 The molecular mass of a compound is found by adding up the atomic masses of all the elements that make it, multiplied by the number of times each atom appears. For sodium chloride, NaCl, this is one each, so the molecular mass is one times the Ar of sodium, 23, plus one times the Ar of chlorine, 35.5. What is the molecular mass of sodium chloride? Pause the video whilst you work it out. The molecular mass of sodium chloride is 58.5 grams per mole. Did you get that right? Don't forget your units. Moles say only how many particles there are, not how much is in them. If one mole of chlorine molecules and two moles of sodium atoms react, they make two moles of sodium chloride atoms. Like this. 2 add 2 really does equal 2, at least when two atoms are combining, to make two-end molecules. Similarly, two molecules of magnesium and one mole of oxygen react to create only two moles of magnesium oxide. If this seems hard, watch our video, Balancing Equations. We can use molecular mass of a substance to calculate the number of moles in a given mass. Let's say we have 1.5 grams of sodium iodide, NaI. How many moles is that? The relative atomic mass, or Ar of sodium, is 23 grams per mole. The relative atomic mass, or Ar of iodine, is 127 grams per mole. Thus, the molecular mass of sodium iodide is 23 grams per mole, plus 127 grams per mole, which is 150 grams per mole. You may notice the molecular mass units are grams per mole. We can work out how many moles we have by putting in the mass and rearranging. This is also known as a triangular equation. You can write it like this. To find any one quantity, cover it up with your finger to give the rearranged equations. We want to find moles. There are 0.01 moles of sodium iodide. In chemistry, we use moles to work out how much of each reaction to weigh out. This means we can make sure we get enough product and none of the reactants are wasted. It also makes it easier to get the product pure. Let's say you need 0.3 moles of silicon dioxide. The relative atomic masses of silicon and oxygen are 28 and 16 grams per mole. What mass do you need? Pause the video whilst you do your calculation. Ready for the answer? The M.R. of silicon dioxide is 28 plus 16 plus 16, which is 60 grams per mole. You will need 18 grams of silicon dioxide to get 0.3 moles.