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# OCR A2 CHEMISTRY F324 and F325- 14th and 22nd June 2016- OFFICIAL THREAD watch

Ka = 10^-pka
pka = -logKa

low/high ka = what
low/high pka = what

which combination means its a strong acid???
2. (Original post by BioStudentx)
I'm struggling to understand! If you post a picture I'll understand better.

A=(B/C) and A=B/C are the same thing? Haha.
Maybe, but If I write a Fraction with a horizontal line, I tend to put A not on that line:

Like imagine this ------- is a line. A fraction has numbers on it and below it. But if the thing the fraction equals is on the line, so for A=B/C it would mean that A/C and B/C are equal because A=B
3. Hardest F325 paper - which year??????
4. (Original post by Jessica Mighall)

Ka = 10^-pka
pka = -logKa

low/high ka = what
low/high pka = what

which combination means its a strong acid???
A high Ka value means a stronger acid as more of it dissociates. A low pKa value means a stronger acid, it is like pH as in the lower it is, the stronger the acid
5. (Original post by Jessica Mighall)

Ka = 10^-pka
pka = -logKa

low/high ka = what
low/high pka = what

which combination means its a strong acid???
High Ka= strong acid
High Ka is low pKa so low pKa is strong acid.

I remember it by thinking about it like pH. A smaller pH means the acid is stronger so a smaller pKa means the acid is stronger.
6. (Original post by lai812matthew)
june 2015 4ciii) how do you get 0.12M in C2H5COOH concentration.
The conc of the 25cm3 sol of ethanoic is 0.48*0.025 but then it is diluted up to 100cm3 so divide by 0.1

(0.48*0.025)/0.1 = 0.12
7. (Original post by lai812matthew)
june 2015 4ciii) how do you get 0.12M in C2H5COOH concentration.
n = cv, n = 0.48x0.035
n = 0.012mol

c = n/v
Total volume = 0.1
c = 0.012/0.1
c = 0.12moldm^-3
8. (Original post by GawdHeresEmmie)
The conc of the 25cm3 sol of ethanoic is 0.48*0.025 but then it is diluted up to 100cm3 so divide by 0.1

(0.48*0.025)/0.1 = 0.12
I just did this and I expected there to be more to it, the fact she filled the water up to 100.0, had no relevance?
9. why ph in a standard hydrogen half cell=0?????
10. What's been the highest mark/100 for this paper ever for 90 UMS?
11. (Original post by lai812matthew)
why ph in a standard hydrogen half cell=0?????
If you have -log1 = 0

It's just hydrogen, so the pH is the highest it can get.
12. (Original post by lai812matthew)
why ph in a standard hydrogen half cell=0?????
Standard concentrations are 1 mol dm-3 so pH = -log(1) = 0
13. do you need to know the equations for hydrogen-oxygen fuel cells?
14. (Original post by oxfmarfa1)
do you need to know the equations for hydrogen-oxygen fuel cells?
I think so, I saw a question come up about it earlier in this thread.
15. (Original post by lai812matthew)
why ph in a standard hydrogen half cell=0?????
-log(1)
cos [H+] = 1 moldm-3
idk if you do maths or what, but any logarithm with a 1 ln(1), log(1) etc = 0 because x^0 = 1
16. (Original post by Kamara7)
What's been the highest mark/100 for this paper ever for 90 UMS?
I'm guessing you mean 90% of the available UMS so 135 UMS? If that's the case then last year it was the highest ever with 87/100. But it was a nice paper.
17. (Original post by Dinasaurus)
If you have -log1 = 0

It's just hydrogen, so the pH is the highest it can get.
You can get negative pH, imagine a solution of HCl with a concentration of 50moldm^-3, the [H+] is gonna be 50 and -Log(50) is negative
18. (Original post by Kamara7)
What's been the highest mark/100 for this paper ever for 90 UMS?
If you mean 90% UMS, it was last year at 87/100 for an A*

If you mean 90 (a C), it was Jan 12 and June 15 at 63

EDIT: see here https://147a51c371676299402810da40fa...y%20A%20A2.pdf
19. (Original post by JennyMoorcroft)
Hardest F325 paper - which year??????
jan 2013
20. (Original post by oxfmarfa1)
do you need to know the equations for hydrogen-oxygen fuel cells?
yes

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