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

http://www.ocr.org.uk/Images/79734-question-paper-unit-f325-equilibria-energetics-and-elements.pdf

Can anyone explain question 3 (e) (ii)

I get that Cl- is in a 1:2 ratio with the complex but I don't understand how this shows there's only 1 Cl- in the complex??
Can anyone explains this please?

Thanks, I think I understand what you mean. I've never seen a buffer question like that one before, need more practice with ones like that

From my latest mock

Ignore the scribbles (they're wrong anyway)

This question came up whilst I was revising, and I thought you might find it helpful as it's similar to the one I helped you with:

Shouldn't it be chromium? The solid CrCl₃

6 moles of water of crystallization so
CrCl₃.6H₂O?

Ah you already knew the answer?

What is the white ppt btw? Is it some sort of silver hydroxide?

AgCl

It was a way of working out the compounds lol

what paper is that?

AgCl

It was a way of working out the compounds lol

Do you remember you asking a similar problem that involved chloride ions and silver? It's the same principle.

Silver chloride has the formula AgCl so an ionic equation will be
Ag⁺ + Cl^- > AgCl

Working out the ratio like you've done gives 1:2 but you know that the reaction is 1:1 for the silver ppt so 2Cl- needs to be present outside of the complex. With complex ions, a ligand is unlikely to be involved with a precipitation reaction so you know that the 2Cl- isn't connected by coordinate bonds (as a ligand) and must be outside of the complex.

Hence, the only complex to have 2Cl- outside is B.

What is C?

This question came up whilst I was revising, and I thought you might find it helpful as it's similar to the one I helped you with:

AgCl

It was given as a hint to help us work out the anion in the compound. Now, from AS, we know that Cl- reacts with Ag+ (in AgNO3) to form a white precipitate right?

So that's what we'd use to work out the formula of the compound

Sorry, I meant what is D?

You know for the second question, I understand why the HCOONa decreases by 5x10-4 but why does the acid increases by 5x10-4