Buffer Calculations in Exam Questions for OCR

[@amirbatayi3750]

I’m glad that I found your channel as there aren’t many chemistry teachers that are ocr specific and explain things clearly.

[@mrmurraygreen]

I’m very glad you’re finding it useful! Good luck in your exams 👏

[@KJ-ix8zh]

This is such a good video, I appreciate the way you explain your thought process of each step and detail the method and reasoning as that’s where I struggle the most with deciphering mark schemes.

[@kameronchander]

I met a girl at the UOM open day who recommended your channel to me, was not disappointed! she said she was one of your past students, can't remember her name though!

[@mrmurraygreen]

amazing! welcome to the channel

[@TheRealDanielTV]

Hi Mr Murray, I dont really understand why youre allowed to use moles instead of concentration, when using the Ka formula? Please could you rephrase your explanation as I am quite lost. Thanks

[@mrmurraygreen]

Hi, very sorry, I missed your comment until just now. When I rearrange the Ka expression as a subject of [H+] for a buffer, I find myself with the [HA] on the top (numerator) and the [A-] on the bottom (denominator). They are both concentration terms which mean they are both 'moles divided by volume'. Since they are in the same solution, these volume terms would be the same (and cancel out) so I can leave them out and just use moles instead.

[@neithonosmani9783]

Thank you, this was really helpful. For the last question, why is it quicker to use moles, isn’t the concentration already stated in the question?

[@mrmurraygreen]

Hi, the concentrations started in the question are not valid for the buffer as when you mix the two solutions together the total volume will change which would change each concentration (c = n/v). What you would need to do is re-calculate the adjusted concentrations to account for the change in the total volume now that the solutions are merged. Since this takes time and the volume terms would cancel out anyway, it is easier just to use moles.

[@daisya7325]

@@mrmurraygreen Hey great video! Although, in the first question it says that “the student assumes that the volume of the solution does not change”. Does that mean we could have just plugged in the concentration values into that equation rather than using moles as the total volume hasn’t changed?

[@mrmurraygreen]

Hi, but the sodium salt isn’t a concentration? It’s a mass. It needs converting into moles to be used. Also sorry for the slow reply, the KZbin studio app is rubbish and keeps hiding comments.

[@neithonosmani9783]

@@mrmurraygreen I understand now, thank you

[@daisya7325]

@@mrmurraygreen Oh right that’s true😂, thanks for the clarification :)

[@Beautifulday1231r]

I don' get why do we sometimes have to minus concentration of salt from conc of acid in order to find conc of buffer? Why do we sometimes can't straight up use this equation

[@mrmurraygreen]

Hello, sorry I appear to have missed your comment. You never subtract the concentration of a salt from the acid, the scenario you are describing sounds like you are substracting the moles of a base from the acid. This is quite common. The reason we have differences is because you may be presented with 2 different situations. Either the examiner is feeling kind, and decides to give you acid and salt data in which case you can very often just put the moles in. Or the examiner may give you acid and base data and to determine the salt and the new acid moles you have to do some ground work with the numbers.

[@IsaacRayMusic]

cheeky sods giving that as a one marker

[@mrmurraygreen]

could not agree more. Similar for Hess cycle questions!

[@rosh_313]

Great video

[@mrmurraygreen]

Thanks!

[@RukkaiahBae]

Awesome

[@ItsSapi]

Do you still use square brackets when writing the expressions even though u use the mole values?

[@mrmurraygreen]

Hi, once you’ve started the calculation it won’t matter what brackets you use. Just make sure when writing the initial Ka (or Kc) expression you use square ones

[@Kanav285]

hi for q1 if i proved that the given solutions give us ph of 4.53 wold that be fine

[@mrmurraygreen]

If you’re struggling to get to the exact answer - check you are not rounding up some of your data too much.

[@thevinecompany7733]

Hi sir I'm doing ocr paper 1 monday, am pretty confident on most topics except transition metals... I dont really understand how to know some of the products and equations even after watching videos. Could you do a video on transition metal questions and explainig them for ocr?

[@mrmurraygreen]

Hi, here are some suggestions for you in case you missed them ... This video covers all of the copper equations = kzbin.info/www/bejne/g3bIdXyEZ7KgsJo This video covers iron and manganese equation = kzbin.info/www/bejne/gYnMaYBjmsiqd5I this video covers some transition metal exam questions = kzbin.info/www/bejne/p2qyZ3muoM-LjK8 HAPPY REVISING!

[@thevinecompany7733]

@@mrmurraygreen Oh right I'll give those videos a watch! Thanks very much