As level / a level chemistry concentration help!!

kdb171997

The table shows the student’s results.
Titration 1 Titration 2 Titration 3 Titration 4 Titration 5
Volume of 13.50 12.10 11.10 12.15 12.15
citric acid
in Cm3

The equation for the reaction is:
C6H8O7 + 3 NaOH ⟶ C6H5O7Na3 + 3 H2O

The concentration of the sodium hydroxide was 0.102 mol / dm3
Concordant results are those within 0.10 cm3 of each other.

Calculate the concentration of the citric acid in mol / dm3
Use only the concordant results from the table in your calculation.
You must show your working. (5)
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Reply 1

AliceV_647

Do you know the volume of NaOH? (I can't seem to work out moles otherwise?)

(edited 4 years ago)

Reply 2

kdb171997

Original post by AliceV_647

Do you know the volume of NaOH? (I can't seem to work out moles otherwise?)

Nope it is not given, I know we we can work out the volume of the citric acid by dividing by 1000 for each titration

Reply 3

kdb171997

Original post by kdb171997

Nope it is not given, I know we we can work out the volume of the citric acid by dividing by 1000 for each titration

Original post by AliceV_647

Do you know the volume of NaOH? (I can't seem to work out moles otherwise?)

Part a states this

A student titrated citric acid with sodium hydroxide solution.
This is the method used.
1. Pipette 25.0 cm3 of sodium hydroxide solution into a conical flask.
2. Add a few drops of thymol blue indicator to the sodium hydroxide solution.
Thymol blue is blue in alkali and yellow in acid.
3. Add citric acid solution from a burette until the end-point was reached.

(Would the volume be 25cm3???? for naoh)

Reply 4

AliceV_647

Original post by kdb171997

Nope it is not given, I know we we can work out the volume of the citric acid by dividing by 1000 for each titration

Hmm... I've got the mean titre for the citric acid (0.01213 dm3 if I'm right?)

Original post by kdb171997

Yep, that's the volume! Give me a sec and I'll see if I can work the rest out :smile:

Reply 5

AliceV_647

Have you got a mark scheme/answer for the question? I've got an answer of 0.070mol.dm3 but obviously I can't be sure that it's right (it's been a few months since I've done any A level chem!)

I'll just type out my workings now :smile:

Reply 6

kdb171997

Original post by AliceV_647

Thanks for your quick replies.. I'm honestly not too sure myself hence I asked on Student room :smile:

There is a link which contains the answer (the last question)
but I can't open the link as it is a DOC file and not a PDF
Could you try to check if the link opens for you
titration_markscheme.doc - singhscience

Reply 7

AliceV_647

Okay, so my workings (again, can't promise this is right!):

25cm3 = 0.025dm3

mol of NaOH = concentration X volume = 0.102 x 0.025 = 2.55x10-3 (0.00255)

therefore mol of citric acid = 2.55x10-3 divided by 3 (as the ratio of NaOH to citric acid is 3:1) = 8.5x10-4

mean titre of citric acid = (12.10 + 12.15 + 12.15) / 3 = 12.13cm3

12.13/1000 = 0.01213 dm3 of citric acid

concentration = mol/vol, therefore conc of citric acid = 8.5x10-4 / 0.01213 = 0.070mol.dm3

No clue if that's the right answer, but that's what I got!

Reply 8

AliceV_647

Original post by kdb171997

Thanks for your quick replies.. I'm honestly not too sure myself hence I asked on Student room :smile:

No problem - I've been going a bit crazy with no work to do since A levels so it's nice to get to use my brain haha!
I'll try to open it now :smile:

Reply 9

kdb171997

Original post by AliceV_647

Seems like a nice and clear response Alice :smile:

Would you explain why you did this step [mean] and why you added these three and not all the volumes please

Reply 10

AliceV_647

Hmm, I can't seem to find it - is it a part of a question (e.g. a), b), c) etc)?

Reply 11

Ðeggs

Original post by kdb171997

Seems like a nice and clear response Alice :smile:

Would you explain why you did this step [mean] and why you added these three and not all the volumes please

You only add concordant titres, defined as titres within 0.1cm^3 of each other.

Reply 12

kdb171997

Original post by AliceV_647

Hmm, I can't seem to find it - is it a part of a question (e.g. a), b), c) etc)?

Here are the questions
http://www.singhscience.co.uk/uploads/2/4/1/3/24135648/titration_questions.pdf

I think it's 1d

Reply 13

AliceV_647

Ah brilliant thank you! I found it - I was right! (Just didn't give enough decimal places... whoops!) Hopefully my screenshot of the mark scheme will show up...

Screenshot 2019-09-14 at 22.03.58.png

Reply 14

kdb171997

Original post by Deggs_14

You only add concordant titres, defined as titres within 0.1cm^3 of each other.

Ahh makes sense! Ty

Reply 15

AliceV_647

Original post by kdb171997

Seems like a nice and clear response Alice :smile:

Would you explain why you did this step [mean] and why you added these three and not all the volumes please

When you're finding the mean titre, you only use concordant titres - these are titres that are within 0.1cm3 of each other. You then find the mean from these and discard all other titres - that way your results are more accurate!

And yay - I'm glad my explanation wasn't completely garbage haha!

Reply 16

kdb171997

Original post by AliceV_647

It seems you could easily be a chem teacher haha! :smile:

Reply 17

AliceV_647

Original post by kdb171997

It seems you could easily be a chem teacher haha! :smile:

Haha, I wish! But thank you :smile:

Reply 18

kdb171997

Original post by AliceV_647

Ah brilliant thank you! I found it - I was right! (Just didn't give enough decimal places... whoops!) Hopefully my screenshot of the mark scheme will show up...

Screenshot 2019-09-14 at 22.03.58.png

Thats perfect! Your a star! I had doubts if I should upload to student room because I didn't expect to get any reps that quick haha, I hope you do well in your chosen career!.. BLESS U