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Edexcel - Chemistry Unit 2 - 4 June 2013

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Reply 1360

all_dolled_up

Good luck everyone :smile:

your best is all you can do!

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Reply 1361

Khurc003

Original post by Hi, How are you ?

Good luck guys =)

Good luck to you too!!!

Can I ask you a question..? i hope i can exlain this properly...?

there is a part of titration questions i always mess up.. eg in the jan 2013 paper... Q20a)iv) I know i shouldve come to tsr to ask earlier but this is really stressin gme out last minute... so if anyone could pleaaaase help out...

Chlorine disinfectants are essentially solutions containing chlorine molecules and
chlorate(I) ions in an equilibrium summarised by the equation
Cl2(aq) + H2O(l) 2H+(aq) + ClO(aq) + Cl(aq) Equation 1
The chlorine content of a disinfectant was determined using the following procedure.
1. 10.0 cm3 of the disinfectant was transferred to a 250 cm3 volumetric flask.
2. Approximately 20 cm3 of nitric acid and 20 cm3 potassium iodide solution (both in
excess) were added to the volumetric flask.
3. The solution in the volumetric flask was made up to the mark with distilled water
and then mixed thoroughly.
4. 10.0 cm3 portions of the solution in the volumetric flask were titrated against a
solution of sodium thiosulfate, concentration 0.109 mol dm. Starch solution was..........

(a) (i) Calculate the number of moles of sodium thiosulfate used in the titration.

(ii) Calculate the number of moles of iodine, I2, that reacted in the titration (step 4).

(iii) Hence state the number of moles of chlorine, Cl2, in 10.0 cm3 of the solution in
the volumetric flask. Here i got 1.48x10-3

(iv) Calculate the concentration of chlorine, in mol dm, in the original disinfectant.
not sure how to get the ans to this part..... mark scheme says you multiply the ans to part iii by 25 and then conc in 100 times this value but i dont understand whyyy Please helpp!

sorry ABOUT THE TYPOS.... IM JUST PANINKING NOW THATS WHY :frown:

(edited 10 years ago)

Good luck everyone.

Original post by edex123

since fluorine is not bonded to a hydrogen atom in fluoromethane

But that is not a condition??
What about ketones,aldehydes??? How do they dissolve without an electropositive hydrogen?

I think its because of the strong solute forces in fluoromethan?

http://www.thestudentroom.co.uk/showthread.php?t=1634651

Reply 1364

Tuya

Original post by Khurc003

Good luck to you too!!!

Can I ask you a question..? i hope i can exlain this properly...?

there is a part of titration questions i always mess up.. eg in the jan 2013 paper... Q20a)iv) I know i shouldve come to tsr to ask earlier but this is really stressin gme out last minute... so if anyone could pleaaaase help out...

Chlorine disinfectants are essentially solutions containing chlorine molecules and
chlorate(I) ions in an equilibrium summarised by the equation
Cl2(aq) + H2O(l) í¯í± 2H+(aq) + ClOí¯í³*(aq) + Clí¯í³*(aq) Equation 1
The chlorine content of a disinfectant was determined using the following procedure.
1. 10.0 cm3 of the disinfectant was transferred to a 250 cm3 volumetric flask.
2. Approximately 20 cm3 of nitric acid and 20 cm3 potassium iodide solution (both in
excess) were added to the volumetric flask.
3. The solution in the volumetric flask was made up to the mark with distilled water
and then mixed thoroughly.
4. 10.0 cm3 portions of the solution in the volumetric flask were titrated against a
solution of sodium thiosulfate, concentration 0.109 mol dmí¯í³*í¯í°. Starch solution was..........

(a) (i) Calculate the number of moles of sodium thiosulfate used in the titration.

(ii) Calculate the number of moles of iodine, I2, that reacted in the titration (step 4).

(iii) Hence state the number of moles of chlorine, Cl2, in 10.0 cm3 of the solution in
the volumetric flask. Here i got 1.48x10-3

(iv) Calculate the concentration of chlorine, in mol dmí¯í³*í¯í°, in the original disinfectant.
not sure how to get the ans to this part..... mark scheme says you multiply the ans to part iii by 25 and then conc in 100 times this value but i dont understand whyyy Please helpp!

sorry ABOUT THE TYPOS.... IM JUST PANINKING NOW THATS WHY :frown:

Because
Conc. is moles over volume
The moles you work out as 6.05 to the -4
The original volume was 25cm3. But you want the conc. in moles per dm3. As 1000cm3 give 1 dm3, you divide the moles by 25 over 1000. This is the same as times by 1000 and multiply by 25.

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Reply 1365

Khurc003

Original post by Tuya

Thankyou soo much for the quick reply! but where did u get 6.05 to the -4 from? :confused:

Reply 1366

Tuya

Original post by Khurc003

Thankyou soo much for the quick reply! but where did u get 6.05 to the -4 from? :confused:

.05 x 24.2 x 10-3 x .5
I have no idea how I got hose either, as I did this a few days back. But form my working I can see that I used the conc. formula I mentioned, and get .5 is because of the ratio to which the ions react I suspect, as there is half as many moles as clo or something.

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Reply 1367

Tuya

Original post by Tuya

I mean divide by 25 in the last sentence

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Reply 1368

Khurc003

Original post by Tuya

I mean divide by 25 in the last sentence

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I think i get it now! Thank you ever so much!!

Reply 1369

Cattey watty

Good luck everyone! Hope it goes well! And thanks to everyone who helped me with my many questions along the way! :biggrin:

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Reply 1370

jollygood

Hey guys how did u find the paper? Hard or easy?

Hard

I couldn't finish. Left like 2 marks. :mad:

Easy

Original post by Sciencefella

Easy

Why does NaCl dissolve in water instead of alcohol?
Why does fullerene dissolve in petrol while graphite and diamond do not?

There was another question about carbon neutral that I don't think I answered right.

Reply 1375

Lili94

compared to other past papers, hard. if you look at june 2011 the grade boundaries were 58 for A but i thought that paper was easier than this one tbh.

Reply 1376

rockstar101

yh for the first time multiple choice was hard for me. i usually get 20/20 but this time god know. btw wtf is gas phase never heard that!!
paper wasnt that bad just abit gay but harder than other papers ive done.