# AQA Chemistry A level Ka Question

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#1
This is a question from the assessment materials AQA released today - I can't find the mark scheme and I can't work out the answer, so any help would be appreciated!

A solution that contains 605 mg of NaHSO4 in 100 cm3 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
weak acid.
State the units of Ka

I got 0.0720, but I think this is wrong.
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3 weeks ago
#2
I got 0.072 aswell!
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3 weeks ago
#3
Was anyone able to do the question about rates of reaction labelled 01, with the phosphinate ions and the graph? It’s quite confusing!
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#4
(Original post by i.76)
I got 0.072 aswell!
I think the wording of the question is confusing me, because it says to find the Ka of the hydrogensulfate ion rather than of the NaHSO4? Does this make a difference to the Ka equation? Because HSO4- dissociates into SO42- and H+, so do you do Ka = [SO4 2-][H+] / [HSO4-] OR Ka = [HSO4-][H+]/NaHSO4]? If you say the conc of HSO4- is equal to the one calculated for NaHSO4 then the first ka works but I'm just confused as to why they would be equal! Thanks!
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3 weeks ago
#5
(Original post by zenosyne)
I think the wording of the question is confusing me, because it says to find the Ka of the hydrogensulfate ion rather than of the NaHSO4? Does this make a difference to the Ka equation? Because HSO4- dissociates into SO42- and H+, so do you do Ka = [SO4 2-][H+] / [HSO4-] OR Ka = [HSO4-][H+]/NaHSO4]? If you say the conc of HSO4- is equal to the one calculated for NaHSO4 then the first ka works but I'm just confused as to why they would be equal! Thanks!
Basically just calculate moles of NaHSO4 and then work out its conc., then work out conc. H+ using ph, Hso4 is a weak acid so its a H+^2 question.
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#6
(Original post by i.76)
Was anyone able to do the question about rates of reaction labelled 01, with the phosphinate ions and the graph? It’s quite confusing!
Did you get 5.15 cm3 s-1?
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#7
(Original post by gogrizz123)
Basically just calculate moles of NaHSO4 and then work out its conc., then work out conc. H+ using ph, Hso4 is a weak acid so its a H+^2 question
Is this using the Ka expression Ka = [HSO4-][H+]/NaHSO4] or Ka = [SO4 2-][H+] / [HSO4-] ?
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3 weeks ago
#8
(Original post by zenosyne)
Did you get 5.15 cm3 s-1?
I’m not too sure how to work it out, how did you do it?
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#9
(Original post by i.76)
I’m not too sure how to work it out, how did you do it?
Tangent to the graph starting at (0,0) then find the gradient? not sure if that's right though
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3 weeks ago
#10
(Original post by zenosyne)
Is this using the Ka expression Ka = [HSO4-][H+]/NaHSO4] or Ka = [SO4 2-][H+] / [HSO4-] ?
The second one because HSO4- is acting as a weak acid, tbh it doesnt really matter because you always get max marks for the correct answer
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3 weeks ago
#11
(Original post by zenosyne)
Tangent to the graph starting at (0,0) then find the gradient? not sure if that's right though
yh draw a tangent at t=0 and work it out from there
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#12
(Original post by gogrizz123)
The second one because HSO4- is acting as a weak acid, tbh it doesnt really matter because you always get max marks for the correct answer
So that would be with the assumption that conc HSO4- = conc NaHSO4? That's the part I don't understand - why are the concentrations equal?
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3 weeks ago
#13
(Original post by zenosyne)
This is a question from the assessment materials AQA released today - I can't find the mark scheme and I can't work out the answer, so any help would be appreciated!

A solution that contains 605 mg of NaHSO4 in 100 cm3 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
weak acid.
State the units of Ka

I got 0.0720, but I think this is wrong.
It was stated that the mark scheme would not be released for these assessment materials publicly. It is only with teachers at the moment.
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3 weeks ago
#14
(Original post by zenosyne)
Tangent to the graph starting at (0,0) then find the gradient? not sure if that's right though
Ahh yes ty I got 5.5 so I think there would be a range of answers
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3 weeks ago
#15
(Original post by zenosyne)
So that would be with the assumption that conc HSO4- = conc NaHSO4? That's the part I don't understand - why are the concentrations equal?
But they’re not equal. The conc of NaHSO4 which you work out is 5.04 x 10-3 and then the conc if H+ is 0.0195. This is equal to the conc of A- bc it’s a weak acid so you can do [H+]^2/ [NaHSO4]
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#16
(Original post by i.76)
But they’re not equal. The conc of NaHSO4 which you work out is 5.04 x 10-3 and then the conc if H+ is 0.0195. This is equal to the conc of A- bc it’s a weak acid so you can do [H+]^2/ [NaHSO4]
Yeah, but if the question is to find the Ka of the weak acid HSO4-, surely you'd have to use the expression Ka = [SO4 2-][H+] / [HSO4-] ?
But then obviously there are only the values to find conc of NaHSO4, leading me to believe the concentrations of HSO4- and NaHSO4 must be linked?
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#17
(Original post by wokirow)
It was stated that the mark scheme would not be released for these assessment materials publicly. It is only with teachers at the moment.
hence why im asking here lol
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3 weeks ago
#18
for the order of reaction I got 1.43 but I don’t think that’s correct. Any help please?
Last edited by i.76; 3 weeks ago
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#19
(Original post by i.76)
for the order of reaction I got 1.43 but I don’t think that’s correct. Any help please?
I got 2 - convert the time into rate by doing 1/T. I think. What did you get for the relationship part?
Last edited by zenosyne; 3 weeks ago
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3 weeks ago
#20
(Original post by zenosyne)
I got 2 - convert the time into rate by doing 1/T. I think. What did you get for the relationship part?
I converted the time into rate by doing 15/T, bc the question says for 15cm3 hydrogen. Hmm it’s quite a tricky one I’m not sure.
For the relationship I got: as the initial concentration of phosphinate ions increases, the time taken to collect 15cm3 hydrogen gas decreases.
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