in this question Kb=4.37*10^-4 and the Molarity=.458

Henderson-Hesselbach equation, I'm thinking.

The Henderson-Hasselbalch equation is for buffers, which this is not.

 

What you need to do is construct an ICE table. Do you know how to do that? if you do, you should also know that the numbers (containing the algebraic "x") at the bottom row, can be arranged to give you an expression for Kb, which you already know. then you can solve that equation for "x", and get a value for the concentration of OH, from which you can calculate the pOH, from which you can get the pH.

 

If you need any help with any of these stages, let me know what's missing and i'll see if I can assist you further

Good job Hermann but also use the function of malority and lean mode in your calculator to get the gain of Ph and the up-take phase super-imposition.

i dont reccomend the use of your calculator's built-in function until you're outside of exam situations. it's all very well to press a button and get an answer from the calculator, but you won't get full marks if you didn't include the workings in an exam.