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

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Original post by medic1010101
What did everyone get for the buffer? My pH was 3 point something but I think it's wrong

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I think go got 10 point something not sure tho
Answer dump:
Q3: -1099 DHhyd
-43.05
Q4 104 Kc dm3mol-1
Q5 ph 2.12
3.43 ph buffer
13.03
ph naoh dilute 10.76

x=4
(edited 7 years ago)
Original post by Vannzzoo
I remember getting 2.3 and 2.82 somewhere and 10 point something


10.46 for the pH of the base?😁

What did people put for whether a pH of 7 was neutral, alkaline or acidic at 60'C?
I wasn't sure what calculation to do
I worked out [H+] for a pH of 7 using the kW from 60 degrees. Then I used 1 as my moles of OH- and put that 7 is alkaline at 60 degrees because my answer for [H+] was less? I'm sure I'm not right. I had to take a guess

I must have missed a stoichiometry adjustment somewhere because I got 881kJmol-1 for the hydration of sulphate calculation
(edited 7 years ago)
Original post by RealistZ
Did anyone get 3.93 as the buffer and around 10.67/76 (cant remeber exactly) for ph of NaOH


Yeah!!!!
Original post by RealistZ
Did anyone get 3.93 as the buffer and around 10.67/76 (cant remeber exactly) for ph of NaOH


I got something like this but cant remeber exactly.
I struggled so badly on the Kw question 😭
What did you guys get for the enthalpy change question?
Original post by KB_97
For the buffer solution question there was 0.05 moles of na(can't remember what it was) and 0.04 moles of the weak acid. Which means all the weak acid was used up. How can that be a buffer solution? I still used those number anyway to calculate a ph but I knew it was wrong. I just didn't know what else to do

I think you were supposed to calculate the moles of the first solution, then use that to calculate the concentration of the 100cm solution,then use that for the buffer pH calculation, well that's what I did at least
Original post by KB_97
For the buffer solution question there was 0.05 moles of na(can't remember what it was) and 0.04 moles of the weak acid. Which means all the weak acid was used up. How can that be a buffer solution? I still used those number anyway to calculate a ph but I knew it was wrong. I just didn't know what else to do


I put the A- excess into a ka equation to try to find the H+ conc? I was clutching at straws though, only used it because we were told the ka or kpa? Probably wrong but that's whati tried to do.
Original post by KB_97
For the buffer solution question there was 0.05 moles of na(can't remember what it was) and 0.04 moles of the weak acid. Which means all the weak acid was used up. How can that be a buffer solution? I still used those number anyway to calculate a ph but I knew it was wrong. I just didn't know what else to do


Because it only dissociates as far as Ka
Did anyone get ph 3.55 for the buffer question?


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Original post by jirapat121
What did you guys get for the enthalpy change question?


-1099
Original post by Vannzzoo
-1099


yess!!!!! sameeee
in the buffer was it just ka x 0.04/0.05
Original post by Vannzzoo
-1099


I got +1000 something… shouldn't it be endothermic since bonds are breaking?
What did you put for the written bit about equilibrium with iodine? 'Why isn't it soluble and observations' and what did you put on the redox qu with Cr3+? Also what was the method for the buffer question? I can't remember my answer but I want to reassure myself that my method was right. And did anyone get 104 for Kc? Seemed a large vakue
Original post by comeonengland1
in the buffer was it just ka x 0.04/0.05


Agreed, 3.43

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Original post by ArthurDayne
Answer dump:
Q3: -1099 DHhyd
-43.05
Q4 104 Kc dm3mol-1
Q5 ph 2.12
3.43 ph buffer
13.03
ph naoh dilute 10.76

x=4


Everything correct, well done

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Original post by ArthurDayne
Answer dump:
Q3: -1099 DHhyd
-43.05
Q4 104 Kc dm3mol-1
Q5 ph 2.12
3.43 ph buffer
13.03
ph naoh dilute 10.76

x=4


Concur though i am wondering if i had 10.77.
I don't recall. Chemistry answers are difficult to remember.
That iodine question messed me up, anyone know why I2 isn't soluble and what happens when u add silver nitrate?
Good paper loved it

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(edited 7 years ago)

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