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

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Original post by HFancy1997
Everything correct, well done

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got exactly the same so happy
Original post by HFancy1997


ka x 0.04/0.05.... lol awwkward when u forgot to square it :biggrin: R.I.P becoming a chemist..... 10/10 get rekt
On the buffer question you were given the concentration of HNO2 (weak acid) and the conc of NaNO2 (conjugate base) surely you could just put those concentrations into the Ka expression. Why did everyone use the moles?
Original post by maisym00
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


I blagged something about the H2O not inducing a dipole in the I-I bond.
The buffer was HNO2 > H+ + NO2- I think
So added H+ moves the position of eqm to the left to remove the excess H+ from solution and form more HNO2
Added OH- reacts with the H+ to form water and the position of eqm moves to the right to restore the H+ ions

I don't think I got the same value as you for Kc. I can't remember what I did get but it wasn't as high as 100.
Original post by Kvothe the arcane
Concur though i am wondering if i had 10.77.
I don't recall. Chemistry answers are difficult to remember.


I think I got 11.something but not sure
Does anyone have a qs/mark breakdown? Or planning to make one?
Original post by maisym00
On the buffer question you were given the concentration of HNO2 (weak acid) and the conc of NaNO2 (conjugate base) surely you could just put those concentrations into the Ka expression. Why did everyone use the moles?


Because it was W amount of X moldm-3 and Y amount of Z moldm-3 so their concentrations in the overall solution are lower. You had to work out the number of moles so that you knew their concentration in the full volume of solution.
Original post by Ohhhhhh
That iodine question messed me up, anyone know why I2 isn't soluble and what happens when u add silver nitrate?


i said I2 is non polar so it won attract the water molecule due to no dipoles .. don't know if its right tbh
Original post by Kingofcastle01
I got +1000 something… shouldn't it be endothermic since bonds are breaking?


Hydration is alwayd exothermic

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Original post by jirapat121
I think I got 11.something but not sure


Same I have a strange feeling I got 11. something as well. What were you supposed to do. I did kw divided by the concentration of NaOH and then -logh+
Reply 3290
Original post by swetu1
i said I2 is non polar so it won attract the water molecule due to no dipoles .. don't know if its right tbh


It gave you the answer in the question I thought, equilibrium lied to the left so it mainly exists in the solid form, 1 mark simple
what was ka for the buffer question?
I am actually disappointed at how the paper wasn't even close to the difficulty that I thought it would be...
The first time I've had a smile on my face while doing an exam this year, very happy with F325!
Went well but I appreciate that it was a mores demanding paper? hopefully got about 90 marks anyway.

For the three step predict reaction, I didn't see that they gave you the rate equation for step 1! I devised three steps that all balanced out to give the overall equation but don't think it was right. Will I lose all of the 3 marks?
Did anyone get x = 4 ?
What did people get for the question where there was equilibrium 1 and 2 and they added 2 different solutions and you had to say what you would observe or something? I had a feeling that I could have worked it out if I had more time but I just got really stuck on it and only wrote a line or two and finished off the other questions I had left

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Original post by fghj1
It gave you the answer in the question I thought, equilibrium lied to the left so it mainly exists in the solid form, 1 mark simple


Highly doubt OCR would give u marks for saying something that's in the question. Usually in the mark scheme they say ignore stuff like that cos it was in the question.
Reply 3298
Original post by Ohhhhhh
Highly doubt OCR would give u marks for saying something that's in the question. Usually in the mark scheme they say ignore stuff like that cos it was in the question.


But you still have to apply that knowledge and it is there to catch people out (which it has done)
Original post by Ohhhhhh
Same I have a strange feeling I got 11. something as well. What were you supposed to do. I did kw divided by the concentration of NaOH and then -logh+


Well first I calculated the mole and then times it by 10 to get 250cm^3 then divide the mol by the volume to get the concentration then use kw. I don't exactly remember how I did it lol

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