# AQA Chemistry A level Ka Question

Watch
Announcements

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.

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.

0

reply

Report

#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!

1

reply

(Original post by

I got 0.072 aswell!

**i.76**)I got 0.072 aswell!

0

reply

Report

#5

(Original post by

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!

**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!

0

reply

(Original post by

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

**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!

0

reply

(Original post by

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

**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

0

reply

Report

#8

(Original post by

Did you get 5.15 cm3 s-1?

**zenosyne**)Did you get 5.15 cm3 s-1?

0

reply

(Original post by

I’m not too sure how to work it out, how did you do it?

**i.76**)I’m not too sure how to work it out, how did you do it?

0

reply

Report

#10

(Original post by

Is this using the Ka expression Ka = [HSO4-][H+]/NaHSO4] or Ka = [SO4 2-][H+] / [HSO4-] ?

**zenosyne**)Is this using the Ka expression Ka = [HSO4-][H+]/NaHSO4] or Ka = [SO4 2-][H+] / [HSO4-] ?

0

reply

Report

#11

(Original post by

Tangent to the graph starting at (0,0) then find the gradient? not sure if that's right though

**zenosyne**)Tangent to the graph starting at (0,0) then find the gradient? not sure if that's right though

0

reply

(Original post by

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

**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

0

reply

Report

#13

(Original post by

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.

**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

Give your answer to three significant figures.

weak acid.

State the units of Ka

I got 0.0720, but I think this is wrong.

0

reply

Report

#14

**zenosyne**)

Tangent to the graph starting at (0,0) then find the gradient? not sure if that's right though

0

reply

Report

#15

(Original post by

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?

**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?

0

reply

(Original post by

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]

**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]

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?

0

reply

(Original post by

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

**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.

0

reply

Report

#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

0

reply

(Original post by

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

**i.76**)for the order of reaction I got 1.43 but I don’t think that’s correct. Any help please?

Last edited by zenosyne; 3 weeks ago

0

reply

Report

#20

(Original post by

I got 2 - convert the time into rate by doing 1/T. I think. What did you get for the relationship part?

**zenosyne**)I got 2 - convert the time into rate by doing 1/T. I think. What did you get for the relationship part?

For the relationship I got: as the initial concentration of phosphinate ions increases, the time taken to collect 15cm3 hydrogen gas decreases.

0

reply

X

### Quick Reply

Back

to top

to top