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

Scroll to see replies

I'm getting 42.9%, what is the answer?

21.6 or 21.7 %

My guess would be that they assume 2 moles of that na2feo4 produced
Think about it fe2 in the LHS
But then that would have needed the whole equation

Reply 781

science rules! :)

Original post by Hydro.ZX

Small post so will probably get ignored, so i'm quoting it.

I know that all the papers are really incredibly similar, lots of borne harber cycles, pH calculations, and gibbs free energy equations- but are there not a few areas you could look at which you find slightly more tricky. Do a few more worksheets on calculations or make sure you can answer long answer questions e.g. of buffers.

Good luck Wed; sounds like your confident :wink:

Reply 782

Jtking3000

Original post by arvin_infinity

1. Artists between the 13th and the 19th Centuries used a green pigment called verdigris. The artists made the pigment by hanging copper foil over boiling vinegar.
A sample of verdigris has the formula [(CH3COO)2Cu]2.Cu(OH)2.xH2O.?Analysis of the sample shows that it contains 16.3% water by mass.?Calculate the value of x in the formula.

answer is 5
Didnt know how to do it! Someone explain plss
Hope we get easier q than this!

I might be talking gibberish but this is how I worked it out:

I found the mr of anhydrous verdigris first which was 460.5

because 16.3% of the the mass was water then 460.5 is 83.7% (100-16.3)of the molecules mass. This means the total mass is 460.5/0.837= 550.18 g/mol

550.18*0.163 = 89.67 g/mol

89.67/18 = 5 H20 molecules/mol

Reply 783

haydyb123

Original post by arvin_infinity

21.6 or 21.7 %

My guess would be that they assume 2 moles of that na2feo4 produced
Think about it fe2 in the LHS
But then that would have needed the whole equation

very good point, I see it now, bit naughty to not have an equation though!

Reply 784

CoventryCity

Original post by Diddy91

Can someone please explain this question to me of the january 2011 exam (2C).
6FeSO4 + 7H2SO4 + Na2Cr2O7 --> 3Fe2(SO4)3 + Cr2(SO4)3 + Na2SO4 + 7H2O and question is show that oxidation and reduction have both taken part in this redoz reaction? Now I understand you are targeting Cr and Fe as these are transition elements. But I am not getting the correct values. My oxidation states are coming in at like +10 and +15!!

Cr has been reduced from +6 to +3

Fe has been oxidised from +2 to +3

In these molecules I think Sulphur is in the +6 oxidation state is the key to working it out

Reply 785

*QueenBeee

Original post by haydyb123

If you have a specific question I'll help you out, as those topics are pretty large to go over completely! :smile:

LOL. ITS rate determining step! cant do it, i always seem to get different compounds....is there some sort of rule??

thank u! x

Reply 786

jontylol

How would you guys answer the question "State how the student would identify the end point of an iodine/thiosulphate titration?"

Im pretty sure its add starch and it'll go from blue-black to colourless, but the mark scheme i has says blue-black to white do NOT allow colourless... It has me a bit confused.

Reply 787

CoventryCity

Original post by *QueenBeee

LOL. ITS rate determining step! cant do it, i always seem to get different compounds....is there some sort of rule??

thank u! x

There is no rules, just make it up. Make sure all your molecules balance up as you do it and make sure your 2 step reaction cancels down to give the overall reaction.
Your molecules from the rate equation are involved in the step 1 (the slowest step), so any molecules that are not involved in step 1 but are in the overall reaction will come in step 2

Also there is more than one possible answer for these sort of Q so just because yours is not the same as the mark scheme doesn't mean it is incorrect.

Reply 788

CoventryCity

Original post by jontylol

No idea

Is the step before is when you react Cu2+ with I-. This forms I2 and CuI(white precipitate)
Maybe this is an explanation :confused:

I would have thought they would allow it goes to colourless though?

Reply 789

jontylol

Original post by CoventryCity

No idea

Is the step before is when you react Cu2+ with I-. This forms I2 and CuI(white precipitate)
Maybe this is an explanation :confused:

I would have thought they would allow it goes to colourless though?

Hmm yeah it is... It says the iodine formed was titrated so i don't know whether that means taken out and titrated or what. Oh well im just gonna go with badly worded question hah :P

Reply 790

lbavfc

Could someone explain to me the changes that take place at each electrode in a hydrogen-oxygen fuel cell? I really can't get my head around it for some reason. Thank you :smile:

Reply 791

arvin_infinity