Ka for a weak acid?
Posted from TSR Mobile
It's because the concentration of H+ has a large component from water in a weak acid.
In neutral water, there are already some H+ ions. Say this amount is 'x'. A strong acid might release the equivalent of 10,000x - hence we ignore the contribution from water and say that 10,000 = 10,001 approximately. A weak acid might release only 5x ions. 5x = 6x is a much worse approximation, hence we can't say that the concentration of H+ is equal to the concentration of the NO2- ion.
In neutral water, there are already some H+ ions. Say this amount is 'x'. A strong acid might release the equivalent of 10,000x - hence we ignore the contribution from water and say that 10,000 = 10,001 approximately. A weak acid might release only 5x ions. 5x = 6x is a much worse approximation, hence we can't say that the concentration of H+ is equal to the concentration of the NO2- ion.
Original post by rambo1168
in the question it says that hno2 is a weak acid so WHY can't we write it as [H+]^2? I've always done that for weak acids :/
Posted from TSR Mobile
Posted from TSR Mobile
If it gives you the acid then you write down [H+][A-] (in this case NO2-. It is just when you do the calculation you ASSUME [H+] = [A-] but if you are asked to write the Ka of a known acid you can and must put down the acid and its conjugate base.
[H+]2 is an approximation that works ONLY when:
1. You start with the solution of acid only (ie, the only source of A- is teh acid dissociation)
2. Amount of H+ from the acid dissociation is orders of magnitude higher than the amount of [H+]2 from the water autodissociation.
So in general it is wrong, it is OK only sometimes.
1. You start with the solution of acid only (ie, the only source of A- is teh acid dissociation)
2. Amount of H+ from the acid dissociation is orders of magnitude higher than the amount of [H+]2 from the water autodissociation.
So in general it is wrong, it is OK only sometimes.
Quick Reply
Related discussions
- acids and bases question
- alevel chemistry question
- AQA Chemistry A level Ka Question
- chemistry aqa a-level question help
- Aqa alevel chemistry acids and bases help
- Problem with buffers in A2 Chemistry
- OCR chemistry weak acid and base question
- How do I answer this hard acids and bases titration questions?
- Buffer solution help!!
- Buffer calculation help
- Chemistry a level buffers questions
- chemistry pH question HELP
- year 13 alevel chemistry
- Buffer calculation question A level chemistry help
- ocr chemistry acid question
- a level chemistry edexcel helpp!!
- Can someone please help me with this question about buffers?
- chemistry - acids
- AQA A Level Chemistry Acids and Bases
- A-level Chemistry Study Group 2022-2023
Latest
Last reply 10 minutes ago
Official University of Edinburgh Applicant Thread for 2024Last reply 10 minutes ago
University of Oxford 2025 Undergraduate Applicants Official ThreadLast reply 13 minutes ago
Official UCL Offer Holders Thread for 2024 entryLast reply 14 minutes ago
Official: University of Leicester A199 (foundation year) 2024 entry applicantsPosted 15 minutes ago
University of Surrey academics back no confidence vote as 'morale is very low'Posted 18 minutes ago
Is SOAS the Right Choice for Comparative Literature MA? Insights Wanted!Last reply 21 minutes ago
Official University of the Arts London Applicant Thread for 2024Last reply 25 minutes ago
WJEC Eduqas English Literature prediction 2024Trending
Last reply 4 days ago
Im confused about this chemistry question, why does it form these productsTrending
Last reply 4 days ago
Im confused about this chemistry question, why does it form these products
