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# OCR Chemistry A F325 Equilibria, Energetics and Elements Wed 13 June 2012 Tweet

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1. Re: OCR Chemistry A F325 Equilibria, Energetics and Elements Wed 13 June 2012
(Original post by Jess!ca)
Need help on a pH question..

A solution of phenol in water has a conc. of 4.7dm-3.
1) Write an expression for Ka of phenol

I got Ka = [C6H40-][H+] / [C6H50H]

That was fine..

2) Calculate the pH of this solution of phenol (5)

I understand how to do the first part..
Find the number of moles by doing m/M = 4.7/94 = 0.05mol dm-3

I understand that Ka = [H+]^2 / [C6H5OH] The MS then says 1.3x10^-10 = [H+]^2/ 0.05

But 0.05 is the number of moles, not the concentration? You are not given a volume..
Also, I'm guessing that 1.3x10^-10 is the Acid Dissociation constant, but it doesn't tell you that in the question :s
Its the concentration. You worked out moles per dm3 from grams per dm3!
It should give the dissociation constant.
2. Re: OCR Chemistry A F325 Equilibria, Energetics and Elements Wed 13 June 2012
(Original post by Killjoy-)
Its the concentration. You worked out moles per dm3 from grams per dm3!
It should give the dissociation constant.

7)a why did they divide by 4 ?
3. Re: OCR Chemistry A F325 Equilibria, Energetics and Elements Wed 13 June 2012
(Original post by Killjoy-)
Its the concentration. You worked out moles per dm3 from grams per dm3!
It should give the dissociation constant.
Ahhh! Yeah, I was beginning to wonder why I had used 4.7 as the mass. That makes sense now.

However, I still don't understand where they got the 1.3x10^-10 from..
4. Re: OCR Chemistry A F325 Equilibria, Energetics and Elements Wed 13 June 2012
(Original post by Jess!ca)
Its a specimen paper, Q2 (c)

Its given the Ka right in the beginning of the question!
5. Re: OCR Chemistry A F325 Equilibria, Energetics and Elements Wed 13 June 2012
(Original post by Issy123)
Its given the Ka right in the beginning of the question!
So it has... My teacher didn't put that part of the question in the pack!!! No wonder...

IT ALL MAKES SENSE NAOW!

6. Re: OCR Chemistry A F325 Equilibria, Energetics and Elements Wed 13 June 2012
(Original post by Jess!ca)
So it has... My teacher didn't put that part of the question in the pack!!! No wonder...

IT ALL MAKES SENSE NAOW!

ahhh! that explains it
7. Re: OCR Chemistry A F325 Equilibria, Energetics and Elements Wed 13 June 2012
For question 7d) on the June 2010 paper, why do you times the number of moles of hydrogen peroxide by its molar mass (34) and 40 i don't get where 40 comes into it?

past paper

mark scheme
8. Re: OCR Chemistry A F325 Equilibria, Energetics and Elements Wed 13 June 2012
(Original post by Jess!ca)
So it has... My teacher didn't put that part of the question in the pack!!! No wonder...

IT ALL MAKES SENSE NAOW!

I think mine made that very mistake actually...
Maybe its the software they use to put the papers together.
9. Re: OCR Chemistry A F325 Equilibria, Energetics and Elements Wed 13 June 2012
(Original post by otrivine)

7)a why did they divide by 4 ?
Start with moles of thiosulphate. Moles of manganese hydroxide is the same.

From 1st equation, there are 4 manganese hydroxide formed from every one oxygen. So divide by 4.

(Original post by Issy123)
For question 7d) on the June 2010 paper, why do you times the number of moles of hydrogen peroxide by its molar mass (34) and 40 i don't get where 40 comes into it?

past paper

mark scheme

Multiplying by 40 is the same as dividing by 0.025 which is just dividing by the volume.
10. Re: OCR Chemistry A F325 Equilibria, Energetics and Elements Wed 13 June 2012
(Original post by Killjoy-)
I think mine made that very mistake actually...
Maybe its the software they use to put the papers together.
Yeah it is, It's happened a few times >_<

Atleast we figured it out anyway
11. Re: OCR Chemistry A F325 Equilibria, Energetics and Elements Wed 13 June 2012
(Original post by fudgesundae)
Start with moles of thiosulphate. Moles of manganese hydroxide is the same.

From 1st equation, there are 4 manganese hydroxide formed from every one oxygen. So divide by 4.

Multiplying by 40 is the same as dividing by 0.025 which is just dividing by the volume.
yes thats what i did i first found the number of moles of thisulphate then i did the ration 2:4 ?
12. Re: OCR Chemistry A F325 Equilibria, Energetics and Elements Wed 13 June 2012
(Original post by -Simon-)
They mean 2b)

It is a question about kinetics and you have to explain why BrO3- is first order using the graph. Standard stuff but the origin bit was a bit unfair to those who haven't done much legacy as they haven't ever really had to mention the origin in the new papers...just remember to stick it in a kinetics question next time if it comes up. The explanation of why is above.
I think it's more mathematics. The 1st order graph is described as concentration is directally proportional to rate. A directly proportional relationship is always shown on the graph as a straight (siagonal) line that passes through the origin. If it doesn't pass through the origin then it can't be directly proportional
13. Re: OCR Chemistry A F325 Equilibria, Energetics and Elements Wed 13 June 2012
stuck with ionic equations (Q9 on f325 June 11), I don't understand how the ionic equations are formed on the ms from the information in the question.

Appreciated if anyone can help
14. Re: OCR Chemistry A F325 Equilibria, Energetics and Elements Wed 13 June 2012
(Original post by otrivine)

7)a why did they divide by 4 ?
They took a short cut (albeit not a particularly quick one)
Look at the equations: you halve and then double moles of thiosulphate before finally quartering them.
15. Re: OCR Chemistry A F325 Equilibria, Energetics and Elements Wed 13 June 2012
(Original post by otrivine)
yes thats what i did i first found the number of moles of thisulphate then i did the ration 2:4 ?
The ratio is 4:1

Work back through the equations.

From the 3rd equation, there are 2 thiosulphate for every one iodine. So divide by 2 to get the moles of Iodine.

Then in the 2nd there are 2 Manganese hydroxide for every 1 iodine. So multiply by two again. So essentially nothing has changed, the moles of manganese hydroxide is equal to the moles of thiosulphate.

Then from the 1st equation, there are 4 manganese hydroxide for every 1 oxygen, so you need to divide by 4.

Overall you have just divided by 4.
16. Re: OCR Chemistry A F325 Equilibria, Energetics and Elements Wed 13 June 2012
(Original post by fudgesundae)

Multiplying by 40 is the same as dividing by 0.025 which is just dividing by the volume.
ahh! thanks! then why don't they just use 0.025 rather than x40 i prefer using 0.025! thanks again
17. Re: OCR Chemistry A F325 Equilibria, Energetics and Elements Wed 13 June 2012
(Original post by fudgesundae)
Start with moles of thiosulphate. Moles of manganese hydroxide is the same.

From 1st equation, there are 4 manganese hydroxide formed from every one oxygen. So divide by 4.

Multiplying by 40 is the same as dividing by 0.025 which is just dividing by the volume.
A guide should be created
18. Re: OCR Chemistry A F325 Equilibria, Energetics and Elements Wed 13 June 2012
(Original post by fudgesundae)
The ratio is 4:1

Work back through the equations.

From the 3rd equation, there are 2 thiosulphate for every one iodine. So divide by 2 to get the moles of Iodine.

Then in the 2nd there are 2 Manganese hydroxide for every 1 iodine. So multiply by two again. So essentially nothing has changed, the moles of manganese hydroxide is equal to the moles of thiosulphate.

Then from the 1st equation, there are 4 manganese hydroxide for every 1 oxygen, so you need to divide by 4.

Overall you have just divided by 4.
oh right so in the mark scheme they did it straight away got it thanks
19. Re: OCR Chemistry A F325 Equilibria, Energetics and Elements Wed 13 June 2012
(Original post by Issy123)
ahh! thanks! then why don't they just use 0.025 rather than x40 i prefer using 0.025! thanks again
haha I know, I just divided by 0.025. They tend to take weird shortcuts.

(Original post by Killjoy-)
A guide should be created
yep, took me a while to figure out why on earth they were multiplying by 40 a couple of days ago and since then everyone has been asking

(Original post by otrivine)
oh right so in the mark scheme they did it straight away got it thanks
Yeh they take loads of shortcuts like that in calculation questions.
20. Re: OCR Chemistry A F325 Equilibria, Energetics and Elements Wed 13 June 2012
When you're working out Kc, don't the total moles at the beginning have to equal the total moles at the end (at equilibrium)?
Last edited by The Illuminati; 12-06-2012 at 11:20.
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