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

1. (Original post by AqsaMx)
Attachment 536159

For the gas Mr I got 62 which I concluded to be oxygen but I can't seem to balance the equations. Would I have to show two moles of oxygen formed or can I just show one because if I do one mole then the equation balances??
You're probably having trouble because the gas produced is SO2 as it has a molecular mass of 64. So the equation is actually:

Cu + 2H2SO4 -----> CuSO4 + SO2 + 2H2O

I'm not sure what the mark scheme would say about putting oxygen as the gas, because O2 has a molecular mass of 32. I'm pretty sure you can't just multiply it by 2 to get 64.

Anyway, hope this helped.
In that case I was the wrong way around. Write out the reaction between the two and work out the moles. I've worked it out that you have twice the amount of moles of AgNO3 than the complex but the product AgCl means that the moles of Ag+ and Cl- must be equal. So the complex must have 2Cl- in it's structure as it has half the number of moles.

What confuses me is where the 2H2O and Cl- are left over from the Cr(H2O)6Cl3 structure given to you. I always thought that the ions on the outside of the complex which are bonded due to the oxidation number of the complex ion would react first, which is why I thought there must be 2Cl- ions on the outside. Anyone else know more about this?
How do you know you have twice the number of moles of AgNO3? :/
When I work out the moles I get 2x10-3 Ag+ and 0.01 Cl-
3. (Original post by AqsaMx)
Attachment 536255

Not sure what colour the indicator would be, by saying its anion is blue, does this mean it's blue in acidic conditions??
Anyone know what colours these pHs would give?
4. (Original post by AqsaMx)
Attachment 536323

Does anyone know how to do (b) and question (8)

Seriously need help with these type of questions !

Anyone know how to do these questions??
5. (Original post by AqsaMx)
How do you know you have twice the number of moles of AgNO3? :/
When I work out the moles I get 2x10-3 Ag+ and 0.01 Cl-
0.01 is the number of moles in 250cm3, 25cm3 was taken from that and used in the titration so you have 10 times less moles, hence 0.001 moles of complex.

0.001:0.002
1:2
6. (Original post by Serine Soul)

I mean the complexes it can be are
- [Cr(H2O)6]3+
- [Cr(H2O)5Cl]2+
- [Cr(H2O)4Cl2]+

And the compound and AgNO3 react in a 1:2 ratio, so 2xCl- , outside the complex ion, must be used up to react with the AgNO3

I feel like I'm missing something
My teacher isn't the best at explaining things lol..
How do you know to multiply the moles of Cl by 2?
7. (Original post by AqsaMx)
How do you know you have twice the number of moles of AgNO3? :/
When I work out the moles I get 2x10-3 Ag+ and 0.01 Cl-
How many moles of the original compound is dissolved in 250cm3? How many moles will be in a 25cm3 portion of this solution?
8. (Original post by Hunnybeebee)
Always think I'm fine with f325 and then find questions like these on tsr???

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It's good to challenge yourself, you never know when OCR will chuck in a crappy question (cough cough 'magic tang')
9. (Original post by Serine Soul)
It's good to challenge yourself, you never know when OCR will chuck in a crappy question (cough cough 'magic tang')
loool i hated that q, thats true i have a feeling that this yrs paper is going to be hard 😭

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10. (Original post by ranz)
loool i hated that q, thats true i have a feeling that this yrs paper is going to be hard 😭

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Probably will, being the last of the spec
11. (Original post by AqsaMx)
Attachment 536343

Really difficult question, anyone know how to do this?
Hey, where are you getting these questions from? They look really good and I don't recognise them!
12. What is everyone getting in past papers? I've left the 2014 papers for the week before the exams. But I've done all of the others + a few legacy papers (which I really recommend). I usually get 79-82 on F325 papers the first time I do them and anywhere from 50-54 on F324 papers. I really need harder questions to practise with the crap OCR can pull :I
13. (Original post by Serine Soul)
It's good to challenge yourself, you never know when OCR will chuck in a crappy question (cough cough 'magic tang')
Anyone know if any other places where I can get f324 (particularly nmr) and f325 hard questions (apart from aqa and already been through physicsandmathstutor.com)

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14. (Original post by Hunnybeebee)
Anyone know if any other places where I can get f324 (particularly nmr) and f325 hard questions (apart from aqa and already been through physicsandmathstutor.com)

Posted from TSR Mobile
Think up a molecule. Look it up on SDBS.
15. Who is retaking unit 1 & 2 ocr chemsitry A ??
What is everyone getting in past papers? I've left the 2014 papers for the week before the exams. But I've done all of the others + a few legacy papers (which I really recommend). I usually get 79-82 on F325 papers the first time I do them and anywhere from 50-54 on F324 papers. I really need harder questions to practise with the crap OCR can pull :I
F324: Usualy 90/90 UMS each time. The raw mark required for this varies quite considerably, from 50/60 in June 2015 to 59/60 in January 2013.

F325: I've only done two papers so far but 131/150 and 135/150 (UMS) so far.
17. (Original post by AqsaMx)
My teacher isn't the best at explaining things lol..
How do you know to multiply the moles of Cl by 2?
You don't have to double the moles of Chloride ions. You need to recognise that silver chloride has the formula AgCl.

Complex ion + AgNO3 -> AgCl

Notice that 1 Ag reacts with 1 Cl

but you have 2 times the moles of AgNO3 which you find out from the titration values. It tells you that they both react completely so the complex ion has to give up 2Cl- otherwise there would be a reactant left over.

Complex ion + 2AgNO3 -> 2AgCl

Which should make it obvious that the complex ion must give up 2 Cl-.

You're given the formula Cr(H2O)6Cl3 which can exist in a few ways which have already been shown. The reaction isn't ligand substitution so the 2Cl- lost must be attached outside the complex ion.
[Cr(H2O)6Cl]2+(Cl-)2 . (H2O)
You don't have to double the moles of Chloride ions. You need to recognise that silver chloride has the formula AgCl.

Complex ion + AgNO3 -> AgCl

Notice that 1 Ag reacts with 1 Cl

but you have 2 times the moles of AgNO3 which you find out from the titration values. It tells you that they both react completely so the complex ion has to give up 2Cl- otherwise there would be a reactant left over.

Complex ion + 2AgNO3 -> 2AgCl

Which should make it obvious that the complex ion must give up 2 Cl-.

You're given the formula Cr(H2O)6Cl3 which can exist in a few ways which have already been shown. The reaction isn't ligand substitution so the 2Cl- lost must be attached outside the complex ion.
[Cr(H2O)6Cl]2+(Cl-)2 . (H2O)
That makes a lot of sense thank you so much
Hey, where are you getting these questions from? They look really good and I don't recognise them!
They're from my college
20. (Original post by marioman)
F324: Usualy 90/90 UMS each time. The raw mark required for this varies quite considerably, from 50/60 in June 2015 to 59/60 in January 2013.

F325: I've only done two papers so far but 131/150 and 135/150 (UMS) so far.
Push a bit harder for a couple more marks in F325. Just one or two raw marks can mean a lot of UMS in the upper boundaries for that paper. Then if you mess up a bit on F324 you can still comfortably get an A*.

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