pH Calculations Summary for A-level Chemistry

0:00 Free resource
1:00 Strong Acids
7:13 Pure water
11:40 Strong Bases
20:33 Weak Acids
26:00 Buffers
36:50 Final Solution pH
41:00 Links
Updates:
1) In Q18 the Ka for methanoic acid is meant to be 1.78 x 10^-4 (like I use in my answer in the video). Sorry about that. I went through tweaking a few numbers before I published my resource and clearly missed one. Never mind eh? I have updated the blank set to reflect this.
Links:
Blank set = drive.google.com/file/d/1NUIJ...
Completed set = drive.google.com/file/d/1TRv1...
The link to my full video catalogue is here = docs.google.com/spreadsheets/...
Topics are organised by OCR module in the correct sequence of the specification.
Please like this video if you use this resource and consider subscribing to stay updated and show your support! Thanks so much everyone.

[@georgiastanford9496]

This is really useful thank you!!

[@mrmurraygreen]

You're welcome!

[@elliesmith7518]

Needs more likes this vid

[@harrygaming9090]

omg. you legendddddd

[@mrmurraygreen]

thanks - hope you find it helpful!

[@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

[@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 :)

[@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

[@scottianrussell]

First

[@alizeh4304]

hi

[@alisha2445]

Hi alizeh 🤣

[@minalhq5762]

hi alizeh