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# OCR B Salters - F335 Exam - 15 June 2011 watch

1. (Original post by pearlover)
thank you
do you get the Kw value for water? can you explain about that abit please?
Ok.

Kw is how you work out the pH of strong bases.

You will be given Kw which I think off the top of my head is 1x10^-14, but that might be wrong and it doesn't matter anyways because they tell you it!

So the basic equation is.

Kw = (H+)(OH-)

They will give you the concentration of the alkali, so for example a question might look like this:

Work out the pH of a 0.1mol dm-3 NaOH solution.

Kw = 1x10^-14.

Step 1: You assume NaOH is fully dissociated so slot the concentration of the alkali in for OH- and you know what Kw is.

1x10^-14 = (H+)x(0.1)

Step 2: Rearrange.

(1x10^-14)/0.1 = H+

Answer = 1x10^-13

Step 3: -log(1x10^-13) to work out pH.

pH = 13

Sorry if that is a bit patronising.
2. (Original post by AGM)
Ok.

Kw is how you work out the pH of strong bases.

You will be given Kw which I think off the top of my head is 1x10^-14, but that might be wrong and it doesn't matter anyways because they tell you it!

So the basic equation is.

Kw = (H+)(OH-)

They will give you the concentration of the alkali, so for example a question might look like this:

Work out the pH of a 0.1mol dm-3 NaOH solution.

Kw = 1x10^-14.

Step 1: You assume NaOH is fully dissociated so slot the concentration of the alkali in for OH- and you know what Kw is.

1x10^-14 = (H+)x(0.1)

Step 2: Rearrange.

(1x10^-14)/0.1 = H+

Answer = 1x10^-13

Step 3: -log(1x10^-13) to work out pH.

pH = 13

Sorry if that is a bit patronising.
oooh thats not patronising at all it makes soo much sense nw! thank youu so much
3. (Original post by JackWoot)
-log([H+]) = 7.4

[H+] = 3.98 x 10^8
just a quick question again.
in the mark scheme it says 10^-8 not 10^8
ermm how does that work...
thank you for the help
4. (Original post by prakruti)
just a quick question again.
in the mark scheme it says 10^-8 not 10^8
ermm how does that work...
thank you for the help
sorry I missed off the negative!
5. (Original post by prakruti)
just a quick question again.
in the mark scheme it says 10^-8 not 10^8
ermm how does that work...
thank you for the help
Try it like so:

-log[H+] = 7.4
log[H+] = -7.4
10^log[H+] = 10^-7.4
[H+] = 10^-7.4
[H+] = 3.98 x 10-8
6. if delta S solution is positive then it can still dissolve in solution provided that entropy is in it's favour. in the exam how would we know that entropy is in it's favour, is it becuase delta S system is positive?

also; if in the exam they show an equation with same number of moles on both sides of the equation and ask what the effect of increasing the pressure would be, is the answer simply "no change"?
7. (Original post by pav)
if delta S solution is positive then it can still dissolve in solution provided that entropy is in it's favour. in the exam how would we know that entropy is in it's favour, is it becuase delta S system is positive?

also; if in the exam they show an equation with same number of moles on both sides of the equation and ask what the effect of increasing the pressure would be, is the answer simply "no change"?
Overall entropy is system + surroundings, so I guess system could be as positive as it wants to be, but if surroundings is even more negative the reaction will not occur spontaneously. So I think it would be delta S total?

If the number of moles of gas are the same then the change would be small, I doubt they'd give us a question like that. However, if they did I suppose you would have to start looking at the size of the molecules?
8. (Original post by zangyray)
Try it like so:

-log[H+] = 7.4
log[H+] = -7.4
10^log[H+] = 10^-7.4
[H+] = 10^-7.4
[H+] = 3.98 x 10-8
I get that now. thankss
9. i was feeling okay about it, not too worried as i am about my others, but then i went into college to do arevision session and now panicking
10. Do you guys time yourself when doing the past papers?
11. This is going to sound reaaally stupid but how do you work out how many hydrogens there are in cyclic hydrocarbons that are stuck together- for some reason I always get it wrong and it's always like the first mark so probably the easiest question on the paper :/
12. IM feeling okay for this exam. Done most of the past papers and it seems like the markschemes have a trend in answers so its best to stick with them.
13. What are the conventions for labelling an energy level diagram there were some in the Jan 11 paper.

Is the y axis labelled energy or enthapy, or does it not matter?

And do you label each energy level, energy level 1, 2, 3 etc.?
14. (Original post by Boompw)
What are the conventions for labelling an energy level diagram there were some in the Jan 11 paper.

Is the y axis labelled energy or enthapy, or does it not matter?

And do you label each energy level, energy level 1, 2, 3 etc.?
Its enthalpy I think /\E.
and no you dont have to label each level.
15. (Original post by *Rainbow*)
This is going to sound reaaally stupid but how do you work out how many hydrogens there are in cyclic hydrocarbons that are stuck together- for some reason I always get it wrong and it's always like the first mark so probably the easiest question on the paper :/
If you mean the benzene molecules then it should be conjugated. For each carbon on the structure look for how many bonds it has attached to it. If it has 4 then it will have no hydrogens and if it has 3 bonds then 1 hydrogen atom.
16. (Original post by JamesPen)
IM feeling okay for this exam. Done most of the past papers and it seems like the markschemes have a trend in answers so its best to stick with them.
So you reckon at this stage it's good to keep doing papers and in effect 'learn' from the mark schemes?

Which are the papers of the same syllabus? Jan 2011, June 2010 and was there a Jan 2010?
17. (Original post by Boompw)
What are the conventions for labelling an energy level diagram there were some in the Jan 11 paper.

Is the y axis labelled energy or enthapy, or does it not matter?

And do you label each energy level, energy level 1, 2, 3 etc.?
For each energy level i usually put n=1, n=2, n=3 and the further they get up the closer.
18. I advise Chemguide for anybody who's struggling with NMR. CI doesn't do a god job of explaining it.
19. (Original post by Jamesrb)
So you reckon at this stage it's good to keep doing papers and in effect 'learn' from the mark schemes?

Which are the papers of the same syllabus? Jan 2011, June 2010 and was there a Jan 2010?
You need to have basic knowledge for what is needed to be written but you also need to be able to write consicely and look out for sentences and works they are expecting in answers rather than waffling on.
20. (Original post by JamesPen)
For each energy level i usually put n=1, n=2, n=3 and the further they get up the closer.
In that answer what specifically were they looking for? Will you be able to post a picture.. e.g a link from google

thanks

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