OCR B Salters - F335 Exam - 15 June 2011

Scroll to see replies

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.

Reply 261

pearlover

Original post by AGM

oooh thats not patronising at all it makes soo much sense nw! thank youu so much :smile:

Reply 262

prakruti

Original post by JackWoot

-log([H+]) = 7.4

[H+] ^ {-} = 10 ^ {7.4}

[H+] = 1 / (10 ^ {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 :smile:

Reply 263

RipperRoo

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 :smile:

sorry I missed off the negative!

Reply 264

zangyray

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 :smile:

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

Reply 265

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"?

Reply 266

AGM

Original post by pav

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?

Reply 267

prakruti

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 :smile:

Reply 268

beckyxo

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 :frown:

Reply 269

Jamesrb

Do you guys time yourself when doing the past papers?

Reply 270

*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 :/

Reply 271

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.

Reply 272

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.?

Reply 273

zahre

Original post by Boompw

Its enthalpy I think /\E.
and no you dont have to label each level.

Reply 274

JamesPen

Original post by *Rainbow*

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.

Reply 275

Jamesrb

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?

Reply 276

JamesPen

Original post by Boompw

For each energy level i usually put n=1, n=2, n=3 and the further they get up the closer.

Reply 277

Conor 419

I advise Chemguide for anybody who's struggling with NMR. CI doesn't do a god job of explaining it.

Reply 278

JamesPen

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. :smile:

Reply 279

ManPowa