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# HELP! OCR A buffer calculation question on specimen paper? Tweet

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1. HELP! OCR A buffer calculation question on specimen paper?
does anyone know how to do question 2d) in the specimen paper
i've even looked at the mark scheme but i'm just confused as to why you'd do that :/

As part of an investigation, a student needed to prepare a buffer solution with a pH value of 8.71. From the Ka value of phenol, the student thought that a mixture of phenol and sodium phenoxide could be used to prepare this buffer solution.
The student decided to use a 0.200 mol dm–3 solution of phenol, mixed with an equal volume of sodium phenoxide.
Use your knowledge of buffer solutions to determine the concentration of sodium phenoxide solution that the student would need to mix with the 0.200 mol dm–3 phenol solution.
Ka = 1.3 × 10–10 mol dm–3

2. Re: HELP! OCR A buffer calculation question on specimen paper?
(Original post by Issy123)
does anyone know how to do question 2d) in the specimen paper
i've even looked at the mark scheme but i'm just confused as to why you'd do that :/

As part of an investigation, a student needed to prepare a buffer solution with a pH value of 8.71. From the Ka value of phenol, the student thought that a mixture of phenol and sodium phenoxide could be used to prepare this buffer solution.
The student decided to use a 0.200 mol dm–3 solution of phenol, mixed with an equal volume of sodium phenoxide.
Use your knowledge of buffer solutions to determine the concentration of sodium phenoxide solution that the student would need to mix with the 0.200 mol dm–3 phenol solution.
Ka = 1.3 × 10–10 mol dm–3

MS is wrong the ans. should be 0.013
Last edited by arvin_infinity; 09-06-2012 at 14:57.
3. Re: HELP! OCR A buffer calculation question on specimen paper?
(Original post by arvin_infinity)
MS is wrong the ans. should be 0.013
ahhh! phew! i got 0.013 following those steps and couldn't understand how the MS got 0.13

thank you!
4. Re: HELP! OCR A buffer calculation question on specimen paper?
I got 0.013 not 0.13
i also got 1.95x10-10 not 1.99x10-10
5. Re: HELP! OCR A buffer calculation question on specimen paper?
(Original post by The Illuminati)
I got 0.013 not 0.13
i also got 1.95x10-10 not 1.99x10-10
same! apparently the MS is wrong
6. Re: HELP! OCR A buffer calculation question on specimen paper?
(Original post by Issy123)
x
Wondered if you can explain 1aiii to me
Last edited by arvin_infinity; 09-06-2012 at 18:39.
7. Re: HELP! OCR A buffer calculation question on specimen paper?
(Original post by arvin_infinity)
Wondered if you can explain 1aiii to me
Hmmm, i've looked at that question a few times, and still don't get it... ive seen a similar q on another past paper but the method to get the answer is completely different.
8. Re: HELP! OCR A buffer calculation question on specimen paper?
(Original post by arvin_infinity)
Wondered if you can explain 1aiii to me

(Original post by Issy123)
Hmmm, i've looked at that question a few times, and still don't get it... ive seen a similar q on another past paper but the method to get the answer is completely different.
You work it out using molar ratios the initial rate given is for HCHO which there are 2 mole. You want the formation of O2 which there is half a mole of.
So for every 2 HCHO made half an O2 will be made. the ratio is 2:0.5 or 4:1 therefore to get the initial rate pf the formation of oxygen you take the initial rate of the formation of HCHO in experiment 1 and divide by 4
1x10-12/4 =2.5x10-13
9. Re: HELP! OCR A buffer calculation question on specimen paper?
(Original post by The Illuminati)
You work it out using molar ratios the initial rate given is for HCHO which there are 2 mole. You want the formation of O2 which there is half a mole of.
So for every 2 HCHO made half an O2 will be made. the ratio is 2:0.5 or 4:1 therefore to get the initial rate pf the formation of oxygen you take the initial rate of the formation of HCHO in experiment 1 and divide by 4
1x10-12/4 =2.5x10-13
+rep (owe you from ages ago)

emm..right after I posted this I managed to do it..just had to read the question
10. Re: HELP! OCR A buffer calculation question on specimen paper?
(Original post by Issy123)
Hmmm, i've looked at that question a few times, and still don't get it... ive seen a similar q on another past paper but the method to get the answer is completely different.
Can you pls post the other one
11. Re: HELP! OCR A buffer calculation question on specimen paper?
(Original post by arvin_infinity)
Can you pls post the other one
sure! i'll take a picture of it and upload it
12. Re: HELP! OCR A buffer calculation question on specimen paper?
(Original post by The Illuminati)
You work it out using molar ratios the initial rate given is for HCHO which there are 2 mole. You want the formation of O2 which there is half a mole of.
So for every 2 HCHO made half an O2 will be made. the ratio is 2:0.5 or 4:1 therefore to get the initial rate pf the formation of oxygen you take the initial rate of the formation of HCHO in experiment 1 and divide by 4
1x10-12/4 =2.5x10-13
ahhh! that makes more sense! thanks will hopefully +rep you tomorrow as i've reached my limit today!
13. Re: HELP! OCR A buffer calculation question on specimen paper?
(Original post by arvin_infinity)
Can you pls post the other one

Sorry about the wait! Btw its from a legacy paper
14. Re: HELP! OCR A buffer calculation question on specimen paper?
(Original post by Issy123)
does anyone know how to do question 2d) in the specimen paper
i've even looked at the mark scheme but i'm just confused as to why you'd do that :/

As part of an investigation, a student needed to prepare a buffer solution with a pH value of 8.71. From the Ka value of phenol, the student thought that a mixture of phenol and sodium phenoxide could be used to prepare this buffer solution.
The student decided to use a 0.200 mol dm–3 solution of phenol, mixed with an equal volume of sodium phenoxide.
Use your knowledge of buffer solutions to determine the concentration of sodium phenoxide solution that the student would need to mix with the 0.200 mol dm–3 phenol solution.
Ka = 1.3 × 10–10 mol dm–3

Could u please explain how u did this? I got to 1.95 x10-9....so far xx
15. Re: HELP! OCR A buffer calculation question on specimen paper?
(Original post by Issy123)

Sorry about the wait! Btw its from a legacy paper
No worries..

can you tell me what it says at the begining of the question cuz somewhere in there should say "rate is measured from the disapperance/apperance of something"
16. Re: HELP! OCR A buffer calculation question on specimen paper?
(Original post by arvin_infinity)
No worries..

can you tell me what it says at the begining of the question cuz somewhere in there should say "rate is measured from the disapperance/apperance of something"
It says:

In an experiment, maltose, was hydrolysed to form glucose. HCL behaves as a catalyst for the reaction.

maltose + water -> glucose

The reaction was carried out several times using different concentrations of maltose and of HCL. the intitial rate of each experiment run was calculated and the results are shown below. (i think the rest is on the pic?)
17. Re: HELP! OCR A buffer calculation question on specimen paper?
(Original post by *Rubina*)
Could u please explain how u did this? I got to 1.95 x10-9....so far xx

you use [H+] = Ka x [Acid]/[Base]

But swap the [H+] and [Base] around.. so the equation is now [Base] = Ka x [Acid]/[H+]

which is

1.3x10-10 x (0.2/1.95x10-10)

= 0.13

hope that helped!
18. Re: HELP! OCR A buffer calculation question on specimen paper?
(Original post by Issy123)
It says:

In an experiment, maltose, was hydrolysed to form glucose. HCL behaves as a catalyst for the reaction.

maltose + water -> glucose

The reaction was carried out several times using different concentrations of maltose and of HCL. the intitial rate of each experiment run was calculated and the results are shown below. (i think the rest is on the pic?)
+rep
Actually I could work that out from the table -silly me

ermm so the ans. is 0.048

ive seen a similar q on another past paper but the method to get the answer is completely different.
FAILED - (same method)
Last edited by arvin_infinity; 11-06-2012 at 02:41.
19. Re: HELP! OCR A buffer calculation question on specimen paper?
(Original post by arvin_infinity)
+rep
Actually I could work that out from the table -silly me

ermm so the ans. is 0.048

(same method)
no it's 2.5x10-13
20. Re: HELP! OCR A buffer calculation question on specimen paper?
(Original post by The Illuminati)
no it's 2.5x10-13
I just checked the mark scheme & the answer is 0.048