AQA Chemistry A level Ka Question

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
Give your answer to three significant figures.
weak acid.
State the units of Ka

I got 0.0720, but I think this is wrong.

Scroll to see replies

I got 0.072 aswell!

Was anyone able to do the question about rates of reaction labelled 01, with the phosphinate ions and the graph? It’s quite confusing!

Reply 3

zenosyne

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!

Reply 4

gogrizz123

Original post by zenosyne

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.

Reply 5

zenosyne

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?

Reply 6

zenosyne

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-] ?

Reply 7

i.76

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?

Reply 8

zenosyne

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

Reply 9

gogrizz123

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

Reply 10

gogrizz123

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

Reply 11

zenosyne

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?

Reply 12

username5472528

Original post by zenosyne

It was stated that the mark scheme would not be released for these assessment materials publicly. It is only with teachers at the moment.

Reply 13

i.76

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

Reply 14

i.76

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]

Reply 15

zenosyne

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?

Reply 16

zenosyne

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

Reply 17

i.76

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for the order of reaction I got 1.43 but I don’t think that’s correct. Any help please?

(edited 3 years ago)

Reply 18

zenosyne

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?

(edited 3 years ago)

Reply 19

i.76

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.