Weak Base pH pOH ka and kb Calculations in MCAT Chemistry

[@mohammadsalah3332]

A good tip that Kaplan teaches in their books is that when you take -log[x*10^-4] you can solve by doing 4 - 0.x to get the pH. Using your example in the video, you'd do -log[4*10^-4] = 4 - 0.4 = 3.6 which is a very close approximation.

[@Leah4sciMCAT]

Thanks for the tip!

[@moussamohammedelamin7990]

thank you so much our teacher Leah your video was so interested

[@Leah4sciMCAT]

You are very welcome!

[@estef1999]

For the last example, how would you identify that the M given is for an acid and not a base? When do you know when you need to find Ka instead of Kb? Also what are the problems where u need to account for 2 H+ or 2 OH-? Thank you!

[@Leah4sciMCAT]

I'm sorry, but I don't offer tutoring through KZbin comments. For help with questions like this and more, I recommend joining the MCAT Study Hall. For more details visit join.mcatstudyhall.com/ or contact me through my website leah4sci.com/contact/

[@alexanderyu565]

For both of the examples you could use the approach of converting Kb to Ka and solving for pH correct? Thanks!

[@Leah4sciMCAT]

No, not for both examples. If given a base, such as NH3, we use the Kb expression because NH3 reacts with water as a base, not as an acid.

[@rohansampat]

So is it always going to be x^2 / Mi ?

[@Leah4sciMCAT]

In this context, yes, assuming the rest is negligible

[@hazwithy9829]

PKa+PKb = 14 seems easier to get a rough answer?

[@Leah4sciMCAT]

Yes if you have or can calculate those values.