So we get the acid ionization constant for this acid is equal to the concentration of hydronium times the concentration of the conjugate base. These two guys divided by the concentration of the conjugate acid for this guy. Now, if you are confused where this expression comes from, check out the link below. Now remember, our goal is to find an expression that we can use to solve for the PH. So which one of these guys determines PH? Well, it's this guy.