 Here's another problem. In this one we've got the reaction between hydrogen gas and iodine gas to give hydrogen iodide. We're told Kc at a particular temperature and we're told that at equilibrium the mixture is found to contain hydrogen at this concentration and hydrogen iodide at this concentration and you're asked to calculate the equilibrium concentration of iodine. So again your first step should be to write down the equilibrium expression. Now all we have to do, we know Kc or Kq, we know the concentration of hydrogen, we know the concentration of hydrogen iodide, we just have to substitute in and rearrange to get the concentration of iodine. If you rearrange this to make iodine the subject, you'll get an expression like this, which will be a square bracket and if you plug that into your calculator you should get 4.8 times 10 to the minus 4 molar. Alright so that's that one solved. We might as well just as an exercise work out the units of this particular Kq. So we've got concentration squared on the top, so it's going to be moles per litre times moles per litre and we have two concentrations on the bottom, which is moles per litre times moles per litre. So you can see that in this case all of the units cancel out and for this reaction Kq is unitless. But as you saw with the other example this is not always the case. However as I said because the units of the equilibrium concentration change from reaction to reaction you will often see the equilibrium constant quoted without units, even if it's for a reaction where it should have units. So people don't get too hung up on the units of the equilibrium constants. But as I said it is good to be able to work them out should you ever need them.