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Acids and Bases help

A mixture of methanoic acid and sodium methanoate in aqueous solution acts as an acidic buffer solution. The equation shows the dissociation of methanoic acid. HCOOH(aq) HCOO− (aq) + H+ (aq) Calculate the mass, in g, of sodium methanoate (HCOONa) that must be added to 25.0 cm3 of 0.100 mol dm−3 methanoic acid to produce a buffer solution with pH = 4.05 at 298 K For methanoic acid, pKa = 3.75 at 298 K

could someone break this down for me please? and the theory behind it.
Would really appreciate it.
Thanks!
Original post by adrigoalevel
A mixture of methanoic acid and sodium methanoate in aqueous solution acts as an acidic buffer solution. The equation shows the dissociation of methanoic acid. HCOOH(aq) HCOO− (aq) + H+ (aq) Calculate the mass, in g, of sodium methanoate (HCOONa) that must be added to 25.0 cm3 of 0.100 mol dm−3 methanoic acid to produce a buffer solution with pH = 4.05 at 298 K For methanoic acid, pKa = 3.75 at 298 K

could someone break this down for me please? and the theory behind it.
Would really appreciate it.
Thanks!

Ok. There are a few things you need to know:

pH and [H^+] are related by the formulae pH = -log[H^+] and [H^+] = 10^-pH.

Ka and pKa are related by the formula pKa = -logKa and Ka = 10^-pKa.

The Ka of a weak acid is given by Ka = [H^+][A^-]/[HA]

Since methanoic acid is a weak acid, you can assume that the concentration of undissociated methanoic acid is exactly 0.100 mol dm^-3, since the extent of dissociation will be very small.

Can you start by calculating the concentration of hydrogen ions in a solution with a pH of 4.05 and converting the given pKa value to the Ka?

Now that you have [H^+], [HA] (the concentration of undissociated methanoic acid) and the Ka, what else can you calculate and how might it be useful?
(edited 12 months ago)
Original post by adrigoalevel
A mixture of methanoic acid and sodium methanoate in aqueous solution acts as an acidic buffer solution. The equation shows the dissociation of methanoic acid. HCOOH(aq) HCOO− (aq) + H+ (aq) Calculate the mass, in g, of sodium methanoate (HCOONa) that must be added to 25.0 cm3 of 0.100 mol dm−3 methanoic acid to produce a buffer solution with pH = 4.05 at 298 K For methanoic acid, pKa = 3.75 at 298 K

could someone break this down for me please? and the theory behind it.
Would really appreciate it.
Thanks!


omg i did this question like 20 mins ago
Reply 3
Original post by TypicalNerd
Ok. There are a few things you need to know:

pH and [H^+] are related by the formulae pH = -log[H^+] and [H^+] = 10^-pH.

Ka and pKa are related by the formula pKa = -logKa and Ka = 10^-pKa.

The Ka of a weak acid is given by Ka = [H^+][A^-]/[HA]

Since methanoic acid is a weak acid, you can assume that the concentration of undissociated methanoic acid is exactly 0.100 mol dm^-3, since the extent of dissociation will be very small.

Can you start by calculating the concentration of hydrogen ions in a solution with a pH of 4.05 and converting the given pKa value to the Ka?

Now that you have [H^+], [HA] (the concentration of undissociated methanoic acid) and the Ka, what else can you calculate and how might it be useful?


Ahh I get it now, tysm, these kind of rules like "Since methanoic acid is a weak acid, you can assume that the concentration of undissociated methanoic acid is exactly 0.100 mol dm^-3, since the extent of dissociation will be very small." - How do you make sure you remember them and use them when you run in to a question? I revise it but don't remember to apply it then become confused when the mark scheme says something I've never seen before. Could you also please check your messages when free? I would really appreciate it. Thanks
Original post by adrigoalevel
Ahh I get it now, tysm, these kind of rules like "Since methanoic acid is a weak acid, you can assume that the concentration of undissociated methanoic acid is exactly 0.100 mol dm^-3, since the extent of dissociation will be very small." - How do you make sure you remember them and use them when you run in to a question? I revise it but don't remember to apply it then become confused when the mark scheme says something I've never seen before. Could you also please check your messages when free? I would really appreciate it. Thanks


I have checked my messages and I have seen the one you have just sent. I’ll take a minute to think about how best to respond to it and how to give you the most helpful advice possible.

Active recall methods such as flashcards are the best ways of learning general rules of thumb.

Unfortunately with these sorts of calculations, the best way forward with them is to look at model answers with some commentary (I may prepare some on the main A level chemistry revision thread) to get an idea of what is going on. Mark schemes are produced with examiners who have already studied chemistry at a higher level in mind, so it is largely assumed that they will understand the content despite the vagueness- this is why model answers with commentary are better. Once you start looking over them and seeing recurring themes in the commentary, you’ll begin to form associations between certain principles in chemistry and why things are done as they are in the calculations.

Sorry I couldn’t give a better answer.
(edited 12 months ago)
Reply 5
Original post by TypicalNerd
I have checked my messages and I have seen the one you have just sent. I’ll take a minute to think about how best to respond to it and how to give you the most helpful advice possible.

Active recall methods such as flashcards are the best ways of learning general rules of thumb.

Unfortunately with these sorts of calculations, the best way forward with them is to look at model answers with some commentary (I may prepare some on the main A level chemistry revision thread) to get an idea of what is going on. Mark schemes are produced with examiners who have already studied chemistry at a higher level in mind, so it is largely assumed that they will understand the content despite the vagueness- this is why model answers with commentary are better. Once you start looking over them and seeing recurring themes in the commentary, you’ll begin to form associations between certain principles in chemistry and why things are done as they are in the calculations.

Sorry I couldn’t give a better answer.

Thank you, I appreciate the advice, are there any youtube channels that go through chemistry questions with commentary that you know of? and no that was a perfect answer!
Original post by adrigoalevel
Thank you, I appreciate the advice, are there any youtube channels that go through chemistry questions with commentary that you know of? and no that was a perfect answer!


I’d say try Eliot Rintoul first, since he does AQA.

If that doesn’t help enough, see if the exam question walkthroughs by MaChemGuy (who is an OCR A teacher) are any good. The content should be mostly the same as with the AQA spec (possibly even identical in the acids and bases topics).

Also, not a YouTube channel, but PhysicsAndMathsTutor has model answers for some of the AQA past papers and these may be worth looking at.

Also, another edit, how could I possibly forget about the resources supplied by the wonderful Chris Clay? https://m.youtube.com/@DocClay1978ALevelChemistry/videos and https://drclays-alevelchemistry.com
(edited 12 months ago)
Reply 7
Original post by TypicalNerd
I’d say try Eliot Rintoul first, since he does AQA.

If that doesn’t help enough, see if the exam question walkthroughs by MaChemGuy (who is an OCR A teacher) are any good. The content should be mostly the same as with the AQA spec (possibly even identical in the acids and bases topics).

Also, not a YouTube channel, but PhysicsAndMathsTutor has model answers for some of the AQA past papers and these may be worth looking at.

Also, another edit, how could I possibly forget about the resources supplied by the wonderful Chris Clay? https://m.youtube.com/@DocClay1978ALevelChemistry/videos and https://drclays-alevelchemistry.com


Thank you so much for your time, I will definitely use these resources! Where can I find the physics and maths tutor model answers?

one more question if you wouldn't mind, I have 7 weeks left, so to prioritise an A/A* I have made flashcards on all the organic mechansisms and making basic ones for the rest of the content, I'm reviewing them and then doing exam questions, am I on the right track? is there anything else I should be doing to expand my knowledge?

Thanks again!
Original post by adrigoalevel
Thank you so much for your time, I will definitely use these resources! Where can I find the physics and maths tutor model answers?

one more question if you wouldn't mind, I have 7 weeks left, so to prioritise an A/A* I have made flashcards on all the organic mechansisms and making basic ones for the rest of the content, I'm reviewing them and then doing exam questions, am I on the right track? is there anything else I should be doing to expand my knowledge?

Thanks again!


https://www.physicsandmathstutor.com/past-papers/a-level-chemistry/

Go onto the page with whichever paper you are revising for and you’ll find both video solutions for some papers and written model answers.

Provided you are using the flashcards correctly (i.e as shown here: https://m.youtube.com/watch?v=qTO0XlihBtw) and the flashcards are made so that they each have the information well summarised and just one relevant question each, your plan should be more than reasonable to see an improvement in your performance.

The only other thing I’d suggest you do is work collaboratively with your classmates and arrange group study sessions. If you can successfully teach a concept to someone else, it’s a strong indicator that you understand it well.

Edit: I haven’t forgotten about that message. Will deal with it later though. I just haven’t helped many who are in a similar situation with their A levels and I’m still working out the best way to give a helpful and constructive answer.
(edited 12 months ago)
Reply 9
Original post by TypicalNerd
https://www.physicsandmathstutor.com/past-papers/a-level-chemistry/

Go onto the page with whichever paper you are revising for and you’ll find both video solutions for some papers and written model answers.

Provided you are using the flashcards correctly (i.e as shown here: https://m.youtube.com/watch?v=qTO0XlihBtw) and the flashcards are made so that they each have the information well summarised and just one relevant question each, your plan should be more than reasonable to see an improvement in your performance.

The only other thing I’d suggest you do is work collaboratively with your classmates and arrange group study sessions. If you can successfully teach a concept to someone else, it’s a strong indicator that you understand it well.

Edit: I haven’t forgotten about that message. Will deal with it later though. I just haven’t helped many who are in a similar situation with their A levels and I’m still working out the best way to give a helpful and constructive answer.


Thank you for your advice man, take care

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