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# ocr a f325 revision thread watch

1. (Original post by otrivine)
thanks

I have a question for you or anyone to answer its a stretch and challenge,

Calculate the pH of a buffer solution that is 0.100 mol dm-3 with respect to sodium methanoate , and 0.200 mol dm-3 with respect to methanoic acid. The Ka of methanoic acid is 1.8x10-4

i GET 3.44 and the answer gets 4.05? what do you or anyone get
Yup I get 3.44 too.
2. (Original post by meg0001)
Can someone explain cis- platin please?
Cis-platin binds to DNA in a fast-dividing cell, alters its structure which cause those cells to stop divding and eventually die out. I think that's all you're required to know
3. (Original post by georgiaaaxo)
i also didn't get this for the same reasons - anyone?!
enthalpy change of solution has a greater value in KF than RbF and that's because not just of the fact enthalpy change of hydration and enthalpy change is more exothermic for KF but also the fact that enthalpy change is has greater effect hence making the enthalpy change of solution more positive. Damn i am bad at explaining. Sorry i tried.
4. (Original post by mollymango)
Yup I get 3.44 too.
Agreed
5. With titration curves, say you start with the acid and add the base, does the curve start at the pH of the acid, and end up at the pH of the base? Is the equivalence point the average of the two pHs, or am I thinking too much about this?
6. Explain the changes that take place at each electrode in a hydrogen-oxygen fuel cell.
7. (Original post by georgiaaaxo)
so wait, do we need to know the equation for dichromate, or just the manganese and the thiosulfate one?
We don't need to know any of those. They're always given in the question.
8. would i be right in saying the only types of complex that can have cis-trans isomerism are:
octahedral with 4 monodentate ligands, then 2 monodentate ligands of another type
square planar complexes with 2 types of ligand (both mono dentate)
octahedral with 2 bidentate ligands, 2 monodentate

and then optical:
octahedral with 3 bidentate ligands
octahedral with 2 bidentate, 2 monodentate

and thats it?
9. (Original post by meg0001)
Can someone explain cis- platin please?
Cis-platin is a square planar complex ion, where the metal ion is Platinum 2+, and the ligands are 2 NH3 groups and 2 Cl- groups. So the formula is [PtCl2(NH3)2] and the overall charge is 0. And it is the cis isomer, so the two Cl- ligands are 90 degrees apart, and the same with NH3 ligands.

When used as an anti-cancer drug, the Cl- ligands are displaced and the complex bonds to 2 nitrogen atoms on the DNA molecule in the cancerous cell. This prevents the cell from being able to replicate and the cell dies.
10. (Original post by A-New-Start)
enthalpy change of solution has a greater value in KF than RbF and that's because not just of the fact enthalpy change of hydration and enthalpy change is more exothermic for KF but also the fact that enthalpy change is has greater effect hence making the enthalpy change of solution more positive. Damn i am bad at explaining. Sorry i tried.
aw dont worry thanks anyway, i kinda get as i read someone's answer above
11. How do you do enthalpy change of neutralisation when there are different moles of your acid and base and when you have more than one mole of h2o?

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12. (Original post by Pride)
We don't need to know any of those. They're always given in the question.
we dont need to know the manganate one or the thiosulfate one by heart?!
oh ok just saw the last part hm i didnt know that, thanks!!
13. can somebody explain why the first electron affinity is exothermic and the 2nd is endothermic ?
14. can someone help me

jan2013 paper question 1a part v,are you meant to knw mass of a nuetron?

8 part c,i have no idea?/

any1 who finds this paper hard,how are we meant to prepare for stretch and challenge questions?
15. (Original post by Joey952)
can somebody explain why the first electron affinity is exothermic and the 2nd is endothermic ?
have you even looked at the ms
16. (Original post by MathsNerd1)
I'd rather prefer one that is like June 2012 as it would mean I'd be scoring in the 90's
Yeah but it means higher grade boundaries.

Rather have a difficult paper with 85/86 for perfect UMS than an easy paper with like 95/100 for perfect UMS.

Fxck that. It's game time brah!

Teacher gave me one last tip for everyone:
1. It doesn't matter if no one else does it or anyone looks at you, if you get a difficult question get a scrap piece of paper.
2. If you can't get it 100% right move and come back
3. You know the answer let your brain think through everything carefully, until you digest the question and start building up a picture.

Think I may do some Chem 4 and Phys 5 Questions now, and then review my Chem 5 Pre-Bed.
17. (Original post by IndyJK9)
With titration curves, say you start with the acid and add the base, does the curve start at the pH of the acid, and end up at the pH of the base? Is the equivalence point the average of the two pHs, or am I thinking too much about this?
I think that is right, but I believe if we are asked to draw it we just need to know the general correct shape - so does it start at a low pH and end at a high pH etc
18. (Original post by IndyJK9)
With titration curves, say you start with the acid and add the base, does the curve start at the pH of the acid, and end up at the pH of the base? Is the equivalence point the average of the two pHs, or am I thinking too much about this?
yes to the first question. Initially you have excess acid, you add more and more base, then eventually it's just base in there, till that's in excess.

Remember pH is really just a measure of conc. of H+/OH-, acids don't really have a pH. It depends on the conc., and tends to be less than 2 for a strong one, around 3+ for a weak one.

the equivalence point is the point in the titration where the solution of one volume has completely reacted with the other volume. So if you like, the volume at the vertical section of the titration curve is the equivalence point.
19. (Original post by A-New-Start)
you do include it in Kc, but it becomes Kw as conc of H20 is constant. The expression you are talking about is the Kstab value where H2O conc is large and constant and hence not needed.
Ohh right cheers
20. (Original post by Joey952)
can somebody explain why the first electron affinity is exothermic and the 2nd is endothermic ?
In first electron affinity- the negative electron is attracted by the nucleus of the atom.

Second EA- the electron is repelled by the -1 ion, the repulsion needs to be overcome which requires energy.

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