pH Calculations Summary for A-level Chemistry

[@OFF-SHORE-l7k]

What a calming voice

[@georgiastanford9496]

This is really useful thank you!!

[@mrmurraygreen]

You're welcome!

[@elliesmith7518]

Needs more likes this vid

[@JK-vm7yo]

You contradict yourself at @30:15. You first said that you can use moles when moles are given and we didnt know the total solution volume. However the example used right after involves a question where we do know the total volume but u say that we are allowed to calculate H+ using moles?? Pls help im so confuised on when to use moles or not?

[@mrmurraygreen]

Hi - Thanks for your comment but I don't contradict myself, please let me explain - what I am trying to do is explain that if you are calculating the pH of buffer you are allowed to just use moles for [HA] and [A-] instead of concentrations. I do talk initially about an example where you don't know the total volume so you aren't able to physically calculate concentrations but then at 34:00 when doing Q19 I reinforce that in any event you can just use moles here whether you know the total volume or not. To summarise: Whether you know the total volume or not, the volume terms within [HA] and [A-] will cancel out so it means you can use just moles of HA and A- to calculate a buffer pH. I hope this helps :)

[@Beautifulday1231r]

For all buffers calculation, which is more beneficial for me to memorize, the formula of Ka=HA/A- or Henderson-Hasselbatch relationship. I find it easier to use Henderson relationship but only WHEN question is asking for pH but what about for all types of question?

[@mrmurraygreen]

I’ll be honest in that I have been teaching this for 16 years and personally never found any worthwhile value in learning the Henderson Hasselbatch equation 🤷‍♂️😆🔵

[@Beautifulday1231r]

@@mrmurraygreen Haha thanks Sir now that equation is carved in my brain sadly… Trying to remember the other one but why won’t formula be same for buffers and weak acid? Why does weak acid one have square root and the other no?

[@lucasonic3461]

Amazing, can you use Ka equation for question 20 and 21 if ka was given? Because I don’t really understand why they give you kw (20) when you don’t need it, is this to confuse you?

[@robertosoyaccino5638]

Hi - thanks for commenting 😄 you can only use Ka for weak acids and there are no weak acids in question 20 or 21 so Ka would not work. In Q20 it is included to mislead you a little - hopefully they wouldn’t do that in the exam 😅

[@lucasonic3461]

Also can you still not use kw equation to find H+ for question 20?

[@mrmurraygreen]

I’ve replied to this in your other question 👍🏻

[@pchanderchannel5466]

W

[@harrygaming9090]

omg. you legendddddd

[@mrmurraygreen]

thanks - hope you find it helpful!

[@_tomas_mellor_8455]

Is this suitable for AQA?

[@mrmurraygreen]

Yes - I’ve taught both 🔵

[@elliesmith7518]

So if the pH was negative it would be ike 10^ --7

[@mrmurraygreen]

I suppose so yes - although I’ve never seen them give a negative ph in an exam Q 🤔

[@pchanderchannel5466]

W

[@alizeh4304]

hi

[@alisha2445]

Hi alizeh 🤣

[@minalhq5762]

hi alizeh

[@scottianrussell]

First