AQA A2 Chemistry - CHEM4 12/06/13

Scroll to see replies

If the answer to something is 2-amino, 3-hydroxy propanoic acid, could you get away with writing 2-hydroxy, 2-amino propanoic acid?

Reply 181

Jesss9

Original post by popnit

If the answer to something is 2-amino, 3-hydroxy propanoic acid, could you get away with writing 2-hydroxy, 2-amino propanoic acid?

Just remember to put them in alphabetical order, I wouldn't risk it because it's not worth losing a mark for!

Reply 182

Xiomara

Original post by Xiomara

Any chance anyone could explain to me question 5 from here?
http://a-levelchemistry.co.uk/AQA%20Chemistry/AQA%20A2%20Chemistry/Unit%204/4.3%20Acids%20and%20Bases/4.3%20%20exam%20questions.rtf
(it starts on page 8)

I really don't understand how to calculate things from pH curves :/

Anyone?

Reply 183

Rose~

Original post by popnit

Could anyone go through this qs pls:

Calculate the pH after the following solutions are mixed together:
15 cm3 of 0.1 moldm-3 HCl and 10 cm3 of 0.1 moldm-3 NaOH

Cheers

Calculate the number of moles of HCl and NaOH in the standard way, I got mol HCl=1.5x10^-3 and mol NaOH=1x10^-3.
Then calculate the number of moles of H+ remaining by subtracting number of moles of NaOH from number of moles HCl, should get 5x10^-4.
When you know the number of moles of H+ remaining you can calculate its concentration since number of moles=5x10^-4 and volume=25cm3 (10+15), plug those into the equation to find concentration and you should get 0.02. Put that in the pH equation and I got 1.70. I have NO IDEA if any of that is right, sorry if it's all wrong!

Reply 184

popnit

Original post by Jesss9

Just remember to put them in alphabetical order, I wouldn't risk it because it's not worth losing a mark for!

Yeah that is true!
Thanks :smile:

Reply 185

popnit

Original post by Rose~

That's exactly right! Thanks I understand it now. :biggrin:

Reply 186

itssochaotic

this thread is pretty quiet considering how soon the exam is :P
anyone have any idea where I can practice structure determination? Its my worst topic!! and the a level chem website has old papers but they use a different data sheet. still worth doing i guess? are there any other practice resources around?

Reply 187

erniiee

I feel solid on everything BUT structure determination..do you guys think a day focusing on this will sort it out?

Also, I'm starting to think Organic Synthesis isn't actually so horrendous - from what I've seen, we aren't really asked to synthesise anything overtly challenging. Is this a gross misconception? :confused:

Reply 188

flyylikejetz

http://filestore.aqa.org.uk/subjects/AQA-CHEM4-QP-JAN12.PDF
Question 4(a) i usually have no problems with acids and bases but i dont get how the pH is 1.77 when i get 1.67 obviously i am wrong but can anyone explain ?? thanks

Reply 189

popnit

Original post by flyylikejetz

I keep getting 1.97!

Posted from TSR Mobile

Reply 190

flyylikejetz

Original post by popnit

I keep getting 1.97!

Posted from TSR Mobile

Moles of HCl = 0.025 * 0.085 = 0.002125 mol

after addition new volume = 125ml

Molarity = moles/vol = 0.002125/0.125 = 0.017 mol dm-3

pH = -log[H+] = 1.77

Reply 191

lifeisgood2012

can someone explain to me about chromotography

Reply 192

cuckoo99

Original post by flyylikejetz

-Find moles of HCL before added to water
-Find Concentration of H+ in Solution after HCL added by doing Moles of HCL / new volume
-Take -log of this concentration
= 1.768
=1.77

Reply 193

cuckoo99

Original post by lifeisgood2012

can someone explain to me about chromotography

Chromotography is used to seperate substances in a mixture so you can indentify the different components

the Types of chromotography they will ask about=
- Paper Chromatography
- Column Chromatography
-Gas-liquid Chromatogrphy
just go revise it though its normaly just a 1 marker topic though, but things can change ;p

Reply 194

lifeisgood2012

Could you explain that to me please??? cos i tried reading the book a million times but i dont understand it

Reply 195

cuckoo99

All types of Chromatography have 2 phases :
A mobile phase- where molecules can move. This is always a liquid or a Gas
A stationary phase- where the molecules can't move. This must be a solid , or a liquid held in a solid
The components in the mixture seperate out as the mobile phase moves through the stationary phase.

now i will start by explaining Column Chromotagraphy:
use: purifying an organic product
how to do it :
pack glass column with an adsorbent(not absorbent) material like Al2O3(coated with water) This will be the stationary phase.

now add the mixture you want to seperate at the top of the column and let it drain down into the AL2O3. You then run a solvent slowly and continiously through the column. This solvent is the mobile phase.

As your organic mixture is washed through the column, its components seperate out according to how soluble they are in the mobile phase and how strongly they are adsorbed onto the stationary phase.

Each different component will spend some time adborbed onto the stationary phase(Al2O3) and some time dissolved in the mobile phase(the solvent) The more soluble each component is in the solvent, the quicker it'll pass through the column

(edited 10 years ago)

Reply 196

itssochaotic

Guys, could someone please help me understand January 2013 question 7b?
reducing agent for conversion of benzene to cyclohexane is given as H2/Ni...is that because benzene is considered a unsaturated molecule? textbook mentions H2/Ni is a reducing agent for c=c bonds...cheers!

Reply 197

itssochaotic

Original post by erniiee

I feel you! structure determination is always hit and miss for me..just try and keep practising, but remember old papers use a diff data sheet, so you gotta practice using the old data sheet too :P

also we just have to *know* AS mechanisms, we dont actually have to draw them right? like electrophillic addition/ free radical substitution..i hope not.

also anyone know if we have to know titration curves with 2 equivelence points?

Reply 198

popnit

Original post by flyylikejetz

Moles of HCl = 0.025 * 0.085 = 0.002125 mol

after addition new volume = 125ml

Molarity = moles/vol = 0.002125/0.125 = 0.017 mol dm-3

pH = -log[H+] = 1.77

Thanks very much!

Reply 199

erniiee

Original post by itssochaotic

Oh that's true! I think I'll just try and do as many questions as I can between now and next week!

Yea as far as I've seen you just need to know the conditions and reagents for AS mechanisms...however I've revised the mechanisms just in case. Haven't done any papers yet apart from the Jan 13 so don't know if they ask for actual mechanisms! I suspect they'll want to save mechanism questions for the A2 reactions though.

From the spec: "understand the typical shape of pH curves for acid–base titrations in all combinations of weak and strong monoprotic
acids and bases".

So no - as they would be diprotic/diacidic!