 So the first thing we need to do is we need to write an equation. So hydrochloric acid is going to in aqueous solution donate its proton to water molecules and in fact we don't really need to draw an equilibrium arrow here because we know that hydrochloric acid is a very strong acid so we're going to get our H3O plus our hydronium ions and we're going to get our chloride ions. Now what we can assume a mild ratio one to one to one to one mild ratio assuming all of this is happening in a solution the concentration of 0.2 if we assume that this is complete means the concentration of both the chloride ions and the hydronium ions will also be 0.2 so this is the value that we want to use so therefore when we do our pH calculation minus the log base 10 concentration of H plus ions which we've said is equivalent to the hydronium ions then this is going to be pH is equal to minus the log base 10 of 0.2 which works out to be 0.698 which rounds up to 0.70. Fairly simple example and we'll have lots of more examples I'm sure for you in class just to get a feel for how these work. Thanks for watching.