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

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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?

Reply 281

AqsaMx

Original post by AqsaMx

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??

Reply 282

thad33

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

Reply 283

AqsaMx

Original post by Serine Soul

I'm getting the same as thad333. Have you asked your tutors about it?

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?

Reply 284

Serine Soul

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?

Reply 285

Serine Soul

It's good to challenge yourself, you never know when OCR will chuck in a crappy question (cough cough 'magic tang')

Reply 286

ranz

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 😭

Posted from TSR Mobile

Reply 287

Serine Soul

Original post by ranz

loool i hated that q, thats true i have a feeling that this yrs paper is going to be hard 😭

Posted from TSR Mobile

Probably will, being the last of the spec :frown:

Reply 288

ToLiveInADream

Original post by AqsaMx

Really difficult question, anyone know how to do this?

Hey, where are you getting these questions from? :smile:

They look really good and I don't recognise them!

Reply 289

ToLiveInADream

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

Reply 290

alow

Think up a molecule. Look it up on SDBS.

Reply 291

erfanullah

Who is retaking unit 1 & 2 ocr chemsitry A ??

Reply 292

marioman

Original post by ToLiveInADream

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.

Reply 293

thad33

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 AgNO₃which 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)₆Cl₃which 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)

Reply 294

AqsaMx

Original post by thad33

That makes a lot of sense thank you so much :smile:

Reply 295

AqsaMx

Original post by ToLiveInADream

Hey, where are you getting these questions from? :smile:

They look really good and I don't recognise them!

They're from my college :smile:

Reply 296

alow

Original post by marioman

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*.

how r ppl getting on with unit 4 tips?

Reply 299

chanda01

Original post by ranz

how r ppl getting on with unit 4 tips?

Unit 4 as in F324?
F324 is a really nice paper as long as you know your stuff. Make sure you know the spec inside out and then practice papers! Practice lots of NMR and Synthesis. Make sure you know which groups can react with which substances so you can apply this in synthesis.