calculating pH question

[hannah]

When you are given an acid and base and are told to calculate the pH, how do you set up the chemical equation for it? Like is there a specific pattern to setting up the equations and knowing when to put H3O or OH on the right side?

[Vincent Tran 3B]

I think that's dependent on the compound that is being placed in the solution. If it is a base then we use Kb and it produces OH. Inversely, if it is an acid, we use Ka and it produces H3O+ generally.

[DylanNgo3F]

I would try not to form too many patterns in your head as there are too many different compound combinations that could be thrown at you. I say the best thing to do is to try to recognize which compounds are acids and bases. You can try to recognize pattern for recognizing the different compounds. But like Vincent said, you know some rules such as Kb being for bases, and Ka being for acids. If they give you Ka but you know you're going to need to find the pOH since the reaction forms [OH-], turn the Ka into Kb to make your solving easier.

[Ivy Zhang 2A]

To add on, when setting up the equations it is also important to identify the conjugate acid-base pairs, so you know whether to add or remove a proton from an acid or base to obtain its respective conjugate acid-base pair.

[charliea100]

If the compound is a base, then this formula can commonly be used: B- +H2O<->OH- + H2O. You can set up an ICE table with the initial concentration given and the pKb value. If the compound is an acid, then this formula can commonly be used: HA +H2O <-> H3O+ + A-. You can then set up an ICE table from those given values and solve for the [H30+]. The pH would then be -log[H3O+]. The pOH would be -log[OH-].