Aqa alevel chemistry acids and bases help
Hi guys! just wondering if someone can explain to me how you would answer the following question.
A solution that contains 605 mg of NaHSO4 in 100 cm° of solution has a pH of 1.72
Calculate the value of Ka for the hydrogensulfate ion (HSO4) that is behaving as a weak acid.
Give your answer to three significant figures.
I’ve linked the PMT mark scheme and I understand all steps besides M4 where you substract the value of [H+] from the [HSO4-] denominator.
MARK SCHEME : https://pmt.physicsandmathstutor.com/download/Chemistry/A-level/Topic-Qs/AQA/Physical-II/1.12-Acids-and-Bases/Set-I/Acids%20and%20Bases%20MS.pdf
The question is 8d.
Any help in understanding why this is is much appreciated
A solution that contains 605 mg of NaHSO4 in 100 cm° of solution has a pH of 1.72
Calculate the value of Ka for the hydrogensulfate ion (HSO4) that is behaving as a weak acid.
Give your answer to three significant figures.
I’ve linked the PMT mark scheme and I understand all steps besides M4 where you substract the value of [H+] from the [HSO4-] denominator.
MARK SCHEME : https://pmt.physicsandmathstutor.com/download/Chemistry/A-level/Topic-Qs/AQA/Physical-II/1.12-Acids-and-Bases/Set-I/Acids%20and%20Bases%20MS.pdf
The question is 8d.
Any help in understanding why this is is much appreciated
(edited 10 months ago)
The 605mg is of the original amount of NaHSO4, so in order to find the concentration of unreacted NaHSO4, you would have to subtract the concentration of the product formed (SO4 2-). (1:1 ratio)
Ill admit, a pretty odd question, took me a while to understand what was going on
Ill admit, a pretty odd question, took me a while to understand what was going on
(edited 10 months ago)
Original post by PixiePresents
The 605mg is of the original amount of NaHSO4, so in order to find the concentration of unreacted NaHSO4, you would have to subtract the concentration of the product formed (SO4 2-). (1:1 ratio)
Ill admit, a pretty odd question, took me a while to understand what was going on
Ill admit, a pretty odd question, took me a while to understand what was going on
hmm, okay i think i sort of understand. because it’s states calculate the Ka for NaHSO4 acting as an acid - we have to assume not all of it does so.
we only care about the portion that acts as an acid - but why does that mean we subtract (SO42-) product?
(edited 10 months ago)
Original post by PixiePresents
The 605mg is of the original amount of NaHSO4, so in order to find the concentration of unreacted NaHSO4, you would have to subtract the concentration of the product formed (SO4 2-). (1:1 ratio)
Ill admit, a pretty odd question, took me a while to understand what was going on
Ill admit, a pretty odd question, took me a while to understand what was going on
oh nvm - you subtract the amount of product formed form the denominator in order to calculate what NaHSO4 remains - and that is what we calculate Ka for.
AHA! thank you - what a god awful question
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