[StrawberryKoi]

(Original post by xthelasthorcrux)
Thanks so much! Let's hope the grade boundaries of our paper aren't as high as the jan ones.

The worst part is there will probably be lots of those annoying suggest questions >.<

[Fahim711]

in those born hater cycles, sum of enthalpies anticlockwise=sum of enthalpies clockwise, but how would you know which is clockwise and anticlockwise. spent so much times trying to figure it out but got nowhere HEEELLLLPPP

[Bright]

I hate it when they ask you to form equations ..

[Conal09]

Do we need to be able to calculate end points? I found a Q on a paper where you had to sketch the titration curve. It gave you volumes and concentrations of the acids and the base, but no endpoint. Then the MS said the endpoint had to be roughly around a certain volume, but I have no idea how you work those out...?

It was on a legacy paper - June 2010, 3ci, unit 2816/01.

Thanks

[JuxtaposedJames]

So predictions for questions to come up?

[Conal09]

(Original post by Fahim711)
in those born hater cycles, sum of enthalpies anticlockwise=sum of enthalpies clockwise, but how would you know which is clockwise and anticlockwise. spent so much times trying to figure it out but got nowhere HEEELLLLPPP

Forget all the signs at first, and just add up all the enthalpies that have arrows going up, and take away the arrows going down. Then you'll get a final answer which will be positive. Then just make it negative and that's the lattice enthalpy.

But don't forget that sometimes you have to multiply the ionisation enthalpy by two if there's two atoms etc. Maybe that's where you're going wrong?

[:):D]

Has anyone done the June 2010 paper, the new specification paper? I reall neeed help with 3b. It says that a line should be drawn through (0,0), I dont really get why?

[Killjoy-]

(Original post by :):D)
Has anyone done the June 2010 paper, the new specification paper? I reall neeed help with 3b. It says that a line should be drawn through (0,0), I dont really get why?

Is this a rate against concentration curve? 2.d)?
If conc of one reagent is zero then you can't have a reaction taking place!

[The Illuminati]

(Original post by :):D)
Has anyone done the June 2010 paper, the new specification paper? I reall neeed help with 3b. It says that a line should be drawn through (0,0), I dont really get why?

Are you sure it's 3b of the June 2010 as that asks you to calculate the pH, no line drawing involved

[The Illuminati]

(Original post by Conal09)
Do we need to be able to calculate end points? I found a Q on a paper where you had to sketch the titration curve. It gave you volumes and concentrations of the acids and the base, but no endpoint. Then the MS said the endpoint had to be roughly around a certain volume, but I have no idea how you work those out...?

It was on a legacy paper - June 2010, 3ci, unit 2816/01.

Thanks

The endpoint of a strong acid and strong base covers a wide range
The endpoint of a strong acid and weak base covers the lower end of the pH scale
The endpoint of a weak acid and strong base covers the higher end of the pH scale

[Jess!ca]

Need help on a pH question..

A solution of phenol in water has a conc. of 4.7dm-3.
1) Write an expression for Ka of phenol

I got Ka = [C6H40-][H+] / [C6H50H]

That was fine..

2) Calculate the pH of this solution of phenol (5)

I understand how to do the first part..
Find the number of moles by doing m/M = 4.7/94 = 0.05mol dm-3

I understand that Ka = [H+]^2 / [C6H5OH] The MS then says 1.3x10^-10 = [H+]^2/ 0.05

But 0.05 is the number of moles, not the concentration? You are not given a volume..
Also, I'm guessing that 1.3x10^-10 is the Acid Dissociation constant, but it doesn't tell you that in the question :s

[Issy123]

For question 7d) on the June 2010 paper, why do you times the number of moles of hydrogen peroxide by its molar mass (34) and 40 i don't get where 40 comes into it?

past paper
http://pdf.ocr.org.uk/download/pp_10..._gce_f325.pdf?


mark scheme
http://pdf.ocr.org.uk/download/ms_10..._gce_f325.pdf?

[otrivine]

http://pdf.ocr.org.uk/download/pp_11..._gce_f325.pdf?

7)a why did they divide by 4 ?

[Issy123]

(Original post by Jess!ca)
Need help on a pH question..

A solution of phenol in water has a conc. of 4.7dm-3.
1) Write an expression for Ka of phenol

I got Ka = [C6H40-][H+] / [C6H50H]

That was fine..

2) Calculate the pH of this solution of phenol (5)

I understand how to do the first part..
Find the number of moles by doing m/M = 4.7/94 = 0.05mol dm-3

I understand that Ka = [H+]^2 / [C6H5OH] The MS then says 1.3x10^-10 = [H+]^2/ 0.05

But 0.05 is the number of moles, not the concentration? You are not given a volume..
Also, I'm guessing that 1.3x10^-10 is the Acid Dissociation constant, but it doesn't tell you that in the question :s

This sounds familiar... what past paper is it from?

[Issy123]

(Original post by otrivine)
http://pdf.ocr.org.uk/download/pp_11..._gce_f325.pdf?

7)a why did they divide by 4 ?

that question is horrible! In fact i hated that whole paper -.-

[Jess!ca]

(Original post by Issy123)
This sounds familiar... what past paper is it from?

I don't know, my teacher just gave us a tonne of Equilibria questions in a pack >-< I'll try and find out..

[Issy123]

(Original post by Jess!ca)
I don't know, my teacher just gave us a tonne of Equilibria questions in a pack >-< I'll try and find out..

same! wait i'll try finding it!

[Jess!ca]

(Original post by Issy123)
This sounds familiar... what past paper is it from?

Its a specimen paper, Q2 (c)

http://pdf.ocr.org.uk/download/asses...unit_f325.pdf?

[-Simon-]

(Original post by The Illuminati)
Are you sure it's 3b of the June 2010 as that asks you to calculate the pH, no line drawing involved

They mean 2b)

It is a question about kinetics and you have to explain why BrO3- is first order using the graph. Standard stuff but the origin bit was a bit unfair to those who haven't done much legacy as they haven't ever really had to mention the origin in the new papers...just remember to stick it in a kinetics question next time if it comes up. The explanation of why is above.

[otrivine]

http://pdf.ocr.org.uk/download/pp_11..._gce_f325.pdf?

7)a why did they divide by 4 ?