 In a previous video, we were looking at the ways of describing solutions in terms of their strength or concentration, and we realized that one of the ways of doing that is to actually start to get a sense of where the equilibrium lies between the reactants and the products when an acid is added to water. In fact, the extent to which acids are ionized is what we use to determine their relative strengths, and we use this with the acid ionization constant, Ka. We can calculate the value of Ka in the same way that we usually do for any equilibrium constant by looking at the concentration of the products divided by the concentration of the reactants. Now if we know that our acid is going to be added to water, and this is going to be in an equilibrium with the hydronium ions and the anion from the acid, we know that water is going to be a liquid, so therefore its concentration will not change, it will remain constant, so the Ka value then is going to simplify to the concentration of the H3O plus ions multiplied by the A minus ions, and then all divided by the concentration of HA. Now what you can see already when we start to look at something like this is A, there's an assumption that's made that the ratio here is 1 to 1, so this works for what we call a mono-protic acid, so an acid which is only going to liberate one proton in solution, so HCl would be an example of that. If we had an example of something like a di-protic acid, so that would be something like H2SO4, then this doesn't work to the same extent, we just have to change our formula a little bit. What we noticed though very quickly is that as we analyse this particular equation, then we know that a strong acid is going to have a high degree of ionisation, and therefore the product concentrations will be high and the concentration of the acid molecules will be low, and as a result of that we will have a value for Ka which is high, and we started to kind of introduce this idea in the previous video, so hopefully this is just a quick review.