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Equilibria, Energetics and Elements (F325) - June 2011 Exam. watch

1. (Original post by Guinea Pig Lover)
moles mno4 = 4.69 x 10-4
moles H2O2, you divide by 2 and times by 5 (from the previous part) = 1.1725x10-3

then you x by 10 because (250/25) = 10

so you get 1.1725x10-2 in 250cm3

x by 40 to get it into dm3

then x by the mr of H202 (34) to get the mass

= 1.1725x10-2 x 40 x 34 = 15.9gdm-3
hmm dilution always panics me :/

so you times by ten because the 250cm^3 has ten times the amount of substance, but this is also the amount of substance as the 25cm3 undiluted because theres not actually any more/less molecules?

could you also work this out by finding the conc of h2o2 and dividing by 10?
2. (Original post by blush.ox)
Completely agree although it was a nightmare and it stayed that way for me .
OCR biol 2day was horrific, now i'm going to need to do really well in chem on wed
3. (Original post by science rules! :))
OCR biol 2day was horrific, now i'm going to need to do really well in chem on wed
chem5 will be similar to jan11
4. (Original post by apo1324)
Does anyone mind explaining buffers to me?
/If it has already been covered here, could someone link me to the post?

Thank you very much.
buffer solution needs high concentrations of weak acid and high concentrations of conjugate base
This can be obtained by an acid-alkali reaction with the acid in excess, this gives large concentration of acid and salt which can dissociate into conjugate base

weak acid dissociates CH3COOH---->CH3COO- + H+ Equilibrium
Salt dissociates CH3COO-Na+ ------>CH3COO- + Na+

If acid is added [H+] this reacts with the conjugate base and the equilibrium shifts left to try and remove the [H+]

If an alkali is added [OH-] this reacts with the [H+] from the weak acid H+ + OH- ----> H20
equilibrium shifts right to replace reacted [H+]

The point of a buffer is to minimise small changes in pH
5. are there any topics ppl think will deffo be on there?
6. (Original post by waecskt)
hmm dilution always panics me :/

so you times by ten because the 250cm^3 has ten times the amount of substance, but this is also the amount of substance as the 25cm3 undiluted because theres not actually any more/less molecules?

could you also work this out by finding the conc of h2o2 and dividing by 10?
Yea this one is strange though.
My teacher said the moles of H202 in the 250cm3 solution is the same as the moles in the undiluted H202. So in the 25cm3 there is the same amount of moles in the 250cm3, it is just the concentration is diluted down as there are now less mol/dm3
7. (Original post by CoventryCity)
buffer solution needs high concentrations of weak acid and high concentrations of conjugate base
This can be obtained by an acid-alkali reaction with the acid in excess, this gives large concentration of acid and salt which can dissociate into conjugate base

weak acid dissociates CH3COOH---->CH3COO- + H+ Equilibrium
Salt dissociates CH3COO-Na+ ------>CH3COO- + Na+

If acid is added [H+] this reacts with the conjugate base and the equilibrium shifts left to try and remove the [H+]

If an alkali is added [OH-] this reacts with the [H+] from the weak acid H+ + OH- ----> H20
equilibrium shifts right to replace reacted [H+]

The point of a buffer is to minimise small changes in pH
Is the weak acid with its conjugate base a different buffer to the salt of the weak acid and its conjugate base? So a weak acid and its conjugate base could be used as a buffer as well as the salt of the weak acid and its conjugate base? (I don't know if that is clear, so sorry about that).

Thanks.
8. (Original post by apo1324)
Is the weak acid with its conjugate base a different buffer to the salt of the weak acid and its conjugate base? So a weak acid and its conjugate base could be used as a buffer as well as the salt of the weak acid and its conjugate base? (I don't know if that is clear, so sorry about that).

Thanks.
No a weak acid can not be a buffer on its own as a weak acid only partially dissociates meaning there will be not a high enough concentration of the conjugate base. That is why the salt is used to create a high concentration of the the conjugate base needed.

I think this is what you meant right?
9. (Original post by CoventryCity)
No a weak acid can not be a buffer on its own as a weak acid only partially dissociates meaning there will be not a high enough concentration of the conjugate base. That is why the salt is used to create a high concentration of the the conjugate base needed.

I think this is what you meant right?
Ohhhh yeah, thanks. How does the salt of the weak acid produce high levels of H+?
10. (Original post by intellectual1)
chem5 will be similar to jan11
how do u know lol
11. does anyone have any notes for this paper?
12. Has anyone got a link to the specification?

Also you see spread 2.1.12 in the heineman text book about the acids. Do we need to know all the rubbish about the discoveries and experiments or just the key defeinitions of a bronsted lowry acid and base and an alkali?
13. (Original post by yesioo)
does anyone have any notes for this paper?
somebody posted some a few pages back http://www.thestudentroom.co.uk/show...&postcount=892
14. Hello there,

My teacher loves to hand out PowerPoints in Chemistry so I might as well hand them out here too. IMO I find them very helpful, just something else to learn from instead of the OCR textbook all the time.

PS:- I am sorry I have to use an external site to upload them but the size of the PowerPoints are too large to be uploaded directly to TSR.

F325 Module 1:

F325 Module 2:

F325 Module 3:

Hope This Helps!
15. (Original post by apo1324)
Ohhhh yeah, thanks. How does the salt of the weak acid produce high levels of H+?
it dosen't. it fully dissociates to produce the salt ion and the metal ion, which we can ignore as it doesn't effect the buffer.
16. (Original post by waecskt)
it dosen't. it fully dissociates to produce the salt ion and the metal ion, which we can ignore as it doesn't effect the buffer.
Where does the high concentration of H+ come from? Or there isn't a high conc, only a weak one due to the weak acid?
17. (Original post by apo1324)
Where does the high concentration of H+ come from? Or there isn't a high conc, only a weak one due to the weak acid?
Yea the buffer solution only contains high concentration of the weak acid and its conjugate base.
When an alkali is added [OH-] the small concentration of [H+] reacts with it. The equilibrium of the dissociation of the weak acid shifts to the right to try and restore the [H+] concentration
18. (Original post by CoventryCity)
Yea the buffer solution only contains high concentration of the weak acid and its conjugate base.
When an alkali is added [OH-] the small concentration of [H+] reacts with it. The equilibrium of the dissociation of the weak acid shifts to the right to try and restore the [H+] concentration
Oh I see, thanks very much all.
19. (Original post by 786girl)
how do u know lol
they're desprate to keep their a stars, due to the media being all "EXAMS ARE EASIEST IN 108 YEARS, MORE STUDENTS ACHIEVED A GRADE THAN ACTUALLY SAT THE EXAM".

so to achieve this they ask off-syllabas questions dressed up as "strech and challenge" (i.e. perm-gate).
20. somebody posted some a few pages back http://www.thestudentroom.co.uk/show...&postcount=892

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