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

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Anyone got any other questions to share, I am so behind and I don't have time to do like every single past paper, so like difficult questions anyone? Anything?
Original post by Dinasaurus
Anyone got any other questions to share, I am so behind and I don't have time to do like every single past paper, so like difficult questions anyone? Anything?


If you're looking for a tough titration question try question 8 on January 2013.
I can just feel this exam is going to be an utter shambles.
Original post by itsConnor_
Ohh thanks so much :smile:

(typo in my initial question, meant 2mol X with 3mol Cr) :smile:
Weird how if you inverse the the molar ratios you get Manganese 54.9 (incorrect) instead of the correct answer Magnesium 24.3. OCR putting in their greatest efforts to convince students they have the right answer when it's wrong.
Original post by Ezexamsalltheway
HI , guys how many of the transition metal aqueous ion colours do we actually need to know. all of them? thought i should better start my revision now :P rip


The main ones are:
Fe2+ (green),
Fe3+ (yellow),
Cu2+ (pale blue)
Co2+ (pink)

Maybe these, but less likely:
Ni2+ (green)
Mn2+ (pale pink)
Cr3+ (green)
Cr (VI) (orange)
V2+ (violet)
Original post by Dinasaurus
yh can someone explain, all I can see is E = E of reduced + E of oxidised.


It doesn't say that the aluminium cell is positive or negative. However, since aluminium is being oxidised, you can assume that it is more negative than the oxygen cell
would we ever get a buffers question with a dipropic acid?
Original post by Rust Cohle
Weird how if you inverse the the molar ratios you get Manganese 54.9 (incorrect) instead of the correct answer Magnesium 24.3. OCR putting in their greatest efforts to convince students they have the right answer when it's wrong.


So the actual answer was Magnesium but Manganese?

Can you explain what to do with the ratio


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Original post by Dinasaurus
Anyone got any other questions to share, I am so behind and I don't have time to do like every single past paper, so like difficult questions anyone? Anything?


If you're gonna do any I'd suggest Jan 2013 and Jan 2011 - they were pretty difficult ones
Original post by HasanAlam
The main ones are:
Fe2+ (green),
Fe3+ (yellow),
Cu2+ (pale blue)
Co2+ (pink)

Maybe these, but less likely:
Ni2+ (green)
Mn2+ (pale pink)
Cr3+ (green)
Cr (VI) (orange)
V2+ (violet)


Thanks but holy potato. Memory banks ... prepare your self!
In terms of buffers... I am so used to explaining addition of acid/alkali in terms of increasing H+ or OH- which leads to an increase in amount of H2O formed or decrease in amount of H+. Will I always get away with this type of explanation? Sometimes the mark scheme explains them in terms of the actual acid e.g. CH3COOH, and CH3COO-. I mean I don't really get the difference. Any help will be appreciated
Original post by Dinasaurus
Higher KC means forward reaction is higher than lower KC, so more products, EQ further right.


Kc<1 means eq. lies to left, vice versa right?
ImageUploadedByStudent Room1466527008.813326.jpgCan someone explain this


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Original post by mechanism
Kc<1 means eq. lies to left, vice versa right?


kc less than 1, means reactants are greater so left yh.
Original post by mechanism
Kc<1 means eq. lies to left, vice versa right?


If Kc<1 Eqn lies to the left

if Kc=1 proportions of products/reactants are the same

Kc>1 Eqn lies to the right
Original post by FluffyCommie
If Kc<1 Eqn lies to the left

if Kc=1 proportions of products/reactants are the same

Kc>1 Eqn lies to the right


Thanks :smile:
Original post by Dinasaurus
kc less than 1, means reactants are greater so left yh.

Thanks!
Can someone help with part d the answer is 0.13mol/dm^3 image.png
Attachment not found
image.png76.8KB
Original post by mechanism
In terms of buffers... I am so used to explaining addition of acid/alkali in terms of increasing H+ or OH- which leads to an increase in amount of H2O formed or decrease in amount of H+. Will I always get away with this type of explanation? Sometimes the mark scheme explains them in terms of the actual acid e.g. CH3COOH, and CH3COO-. I mean I don't really get the difference. Any help will be appreciated


it may be added NH3 like it was in last years paper but they still accepted the H+ + OH-
They'll accept it if you name the acid itself or just show the formation of water it's the same thing essentially
Original post by Dinasaurus
So the actual answer was Magnesium but Manganese?

Can you explain what to do with the ratio


Posted from TSR Mobile


https://i.gyazo.com/49069f767bf5ea95d7e870c6a979a5e2.png
There is question. Since we know that the Chromium gains mass its going from aqeous ---> solid. So write that half equation:

Cr -----> Cr3+ + 3e-

The X electrode loses mass so it goes from solid ----> aqeous. In order to to help you form the half equation, we're told that the aqeous is XSO4 meaning oxidation state of X = +2.

X2+ + 2e- -----> X

-----------------------------------

Now simply create a full equation with the two half equations:

2Cr3+ + 3X ----> 3X2+ + 2Cr

-------------------------------

Moles of Cr we can find since we know its mass (1.456g) and the mr 52. n = m/Mr = 0.028

Apply this to the full equation and you'll find that the moles of X = 0.042 using stoichiometry (molar ratios)

We can know work Mr of X since we have the mass of 1.021g. Mr = m/n = 24.3 = Magnesium

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