 Now let's try using an equilibrium expression to solve a problem. Here we have sulfur dioxide reacting with oxygen to form sulfur trioxide and we've brought this reaction to equilibrium at a fixed temperature and we analysed the concentrations of the species involved and we are given values for those here and the question says to calculate Kc or Keq if you like. So our first step is always to write the equilibrium expression because you can't go anywhere without the equilibrium expression. So we write it down, Keq or Kc if you like and it equals the product's concentration of sulfur trioxide and that has a stoichiometric coefficient of 2 so we raise it to the power of 2 over sulfur, concentration of sulfur dioxide also raised to the power of 2 times the concentration of oxygen. It has a stoichiometric coefficient of 1 so we just leave it as is. So there's our equilibrium expression. Now we're given some information about the concentrations of our three species so the next important thing to do would be to collect the information that you know and then for this problem all we have to do is substitute those values into our equilibrium expression to determine the value of Keq. So and if you plug that into your calculator you will find you should get 11.7 or if we look at our sig figs two of our concentration values here are only to two significant figures so we should really round that down to 12. Okay now just a short side note about the units of equilibrium constants. For a Kc we're dealing in concentrations. All of the things that go into making up your equilibrium constant are concentration values so they all have units of moles per liter. So if we look at this particular one we have okay eq has the concentration of sulfur trioxide squared on the top so each of those concentrations has units of moles per liter. So because it's squared our units on the top are going to be moles per liter times moles per liter. On the bottom we have three concentrations multiplied together sulfur dioxide and sulfur dioxide again and then oxygen and each of those also has units of moles per liter. Okay so this is what the units of this particular Kq look like but as you can see we can do a bit of cancelling because we've got some things on the top and things on the bottom. We're left with essentially one over moles per liter which is the inverse of moles per liter which is the same as liters per mole which you can also write because moles per liter can be abbreviated as big M the inverse liters per mole can be abbreviated as big M to the minus one. So you can either write it as liters per mole or you can write it as big M to the minus one. You will not always be asked to give the units of Kq to some extent they're sort of irrelevant because they differ from reaction to reaction depending on exactly what goes into the equilibrium constant but it is useful to be able to work out the units should you ever need them.