OCR A2 Chemistry

Could anyone please give me some good revision websites for OCR Chemistry for A2. All i know is s-cool which is ok, but i would like something more syallubus specific.

Thanks!

Reply 1

cobra

I've used chem guide, there is slightly more than is needed for ocr but it is very good : http://www.chemguide.co.uk/

Reply 2

ChemGeez

I've used the site:
www.mp-docker.demon.co.uk
It has the specifications for some exam boards including OCR. So just make sure you click on the correct link.

Some revision notes but mainly revision questions!

Reply 3

ness1

can any of u help me 2 calculate the concentration of an acid once its been diluted? gettin a bit confused! sori
thanku x

Reply 4

cobra

ness

can any of u help me 2 calculate the concentration of an acid once its been diluted? gettin a bit confused! sori
thanku x

use volxconc/1000=mols to work out number of moles before dilution then use the rearrangement 1000xmols/vol = conc using the number of mols put into the dilution and the new volume and bobs ur uncle

Reply 5

Unregistered

ness

can any of u help me 2 calculate the concentration of an acid once its been diluted? gettin a bit confused! sori
thanku x

Yeah, sure. Assume you have 10cm3 of HCl of HCl concentration 1mol/dm3.
Check out the "table below":

HCl : Water volumes (cm3)
10 : 0 gives 1 mol/dm3 concentration of HCl
9 : 1 gives 0.9 mol/dm3 concentration of HCl
8 : 2 gives 0.8 mol/dm3 concentration of HCl
7 : 3 gives 0.7 mol/dm3 concentration of HCl

etc..............

Explanation:

You start of with 10 cm3 of 1 mol/dm3 HCl. [1dm3]

- Next you have 9 cm3 of 1mol/dm3 of HCl and this is 0.009 moles per 9 cm3

- However because you added 1 cm3 of water to 9 cm3 of HCl, by volume, you ACTUALLY get 0.009 moles per (9+1) cm3 of HCl.

- This then becomes 0.009 moles per 10cm3 of HCl

- Finally, x100 gives the final concentration to be 0.9 moles per dm3.

Its the same for the other stages. Take 8:2 =>

- 8 cm3 of 1 mol per 1000 cm3 = 0.008 moles of HCl per 8 cm3

- However, you have added an extra 2 cm3 of water so by volume, you get 0.008 moles of HCl per (8+2) cm3

- That is, 0.008 moles per 10 cm3

- x100 gives the final concentration to be 0.8 moles/dm3

The method applies for ALL similar dilution calculations of different concentrations. If you are doing a planning exercise with limited words, you dont have to include how you worked out the different concentrations by diluting, as long as you understand it for yourself.

I admit the dilution of concentrations does seem a bit mind-boggling at first, but it should make immediate sense as soon as its explained to you. If you are still struggling and do not understand my method, then I recommend you get some quick maths lessons. The key is: DO NOT REVISE derivations of an equation/formula, but always UNDERSTAND the DERIVATIONS. What you want to revise are EQUATION FORMULAS, whereas the rest of the derivations that links to it must be done using logic and understanding.

OCR A2 Chemistry

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