OCR A2 CHEMISTRY F324 and F325- 14th and 22nd June 2016- OFFICIAL THREAD

Reply 3420

FluffyCommie

5

Original post by HFancy1997

Everything correct except for the first number, it said 2 sf so 0.029

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Wasn't it also supposed to be given in standard form?

Reply 3421

EricPiphany

17

Original post by Serine Soul

Yes bois

I think I lost a mark on the first one for wrong s.f :/

And the buffer one, you were simply adding x moles NO2-...

Don't worry you'll be fine.

Speaking to people who sat the exam with me, lots of them messed up the buffer question.

I certainly messed up parts. But with high UMS last year, and good UMS in the first exam, I should have the A I need.

Reply 3422

adnan_ja01

1

Original post by Sahil_

It asked whether a ph of 7 is acidic/alkaline at 60 degrees, so using the new Kw, you get that the concentration of oh- is larger than the conc of H+ so it is alkaline

exactly what I got!!

Reply 3423

username2640523

8

Original post by maisym00

For the equilibrium question I put: I- reacts with Ag to form agI (yellow ppt), therefore the position of equilibrium will shift to the left to restore I- conc. As a result, more aqueous iodine was formed so the position of equilibrium of 1 will also shift to the left, therefore more grey/black solid iodine is formed. For the electrochemistry question I needed up with two equations. The oxidising agent one where Cr+3 reacts with AL and the reducing agent one where Cr+3 reacts with feO4- or something. Anyone else?

Yes for thr 6 Marker Half cells q

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Reply 3424

Fmathslad

1

Original post by Vannzzoo

I worked out the moles using the info they gave.
Divided it by 2 because of the mole ratio.
Then multiplied by 2 because of the mole ratio.
Multiplied that by 10 because it was in 250cm3.
Worked out the mr by 2.226 divided by the moles I worked out.
Subtracted the mr of cu(choo)2 from the mr calculated.
Then divided the answer by 18, I got 3.5, so 4=X

the answer was 4.0002 exactly so i don't know how you got 3.5? lucky assumption i guess :biggrin:

Reply 3425

TV1997

1

I think honestly this will be an average paper in terms of grade boundaries, people will have made silly mistakes where they don't expect just like the F324 exam which will pull the marks people think they have got down a bit.

Reply 3426

D1970C

1

Original post by mosheezie

Piss easy FULL UMS FOR ME! My whole college said it was easy af, you guys stop stressing!
Grade boundaries
A* -90
A - 85
Everything below mehh......

hahahaha what a c*unt

Reply 3427

username2221225

7

Original post by BioStudentx

HIO + OH- for step 1, instead of what you got.

H + I- --> HI
HI + H2O2 --> intermediate for given step

Is what I got for Step 1 and 2

Reply 3428

Jsey

Original post by SazahSauce

It was the same shape as water, i swear thats non-linear or bent shaped

Water has bond angles of 104.5 degrees and a shape of bent / non-linear

Reply 3429

FluffyCommie

5

Original post by maisym00

For the equilibrium question I put: I- reacts with Ag to form agI (yellow ppt), therefore the position of equilibrium will shift to the left to restore I- conc. As a result, more aqueous iodine was formed so the position of equilibrium of 1 will also shift to the left, therefore more grey/black solid iodine is formed. For the electrochemistry question I needed up with two equations. The oxidising agent one where Cr+3 reacts with AL and the reducing agent one where Cr+3 reacts with feO4- or something. Anyone else?

That's pretty much what I did, so yay :biggrin:

Reply 3430

four4

2

Anyone going to make an unofficial markscheme?

Reply 3431

cr7alwayz

8

Original post by Jsey

You find the work backwards through the titration to find the moles of Cu2+. Then you multiply the moles of Cu2+ by the molecular mass of Cu(HCOO)2 to find the mass in grams. Then you subtract this value from the grams they gave you in the question. Then you use the remaining grams to find the moles of water. Using moles equals mass over molar mass. Then lastly you divide the moles of water by the moles of the Cu2+ to get the molar ratio.

Damn, I think after finding the moles of Cu2+ I worked out the molar mass of the wholecompund by dividing the mass they gave by moles of Cu2+. Then, I took away molar mass of Cu(HCOO)2 to from that to get the molar mass of water. Then divide by 18 to get 17 for X.
So basically, I used the method on page 220 in the textbook.

Reply 3432

MrZebraCookie

2

Could we draw the diagrams in pencil (the harber cycle and the cell potential)? I'm afraid it wouldnt show up

Reply 3433

adnan_ja01

1

Original post by milemed11

the ph was given as 7 so i worked out the conc of h+ then divided kw by that to give oh-
oh- > h+ so i put alkaline

anyone else?

yess yess

Reply 3434

Brandoncarlin

1

What I got for the 4-step mechanism:

Overall equation: H2O2 + 2I- + 2H+ forms I2 + 2H2O

H2O2 + I- forms IO- + H2O
IO- + H+ forms HIO
HIO + I- forms I2 + OH- (given in question)
OH- + H+ forms H2O

Reply 3435

_NMcC_

15

Original post by Fmathslad

i did it by finding out the concentration of (h+) and the concentration of (oh-) and since the concentration of (oh-) was most than (h+), this shows that it was in fact alkaline, but few others did it your way too so i assume your method might be correct

Yeah same, the OH- concentration was higher than the H+ concentration so it seemed logical to say that the the solution was Alkaline.

Reply 3436

Vannzzoo

2

Original post by Fmathslad

the answer was 4.0002 exactly so i don't know how you got 3.5? lucky assumption i guess :biggrin:

Looooool idk 😭😭

Reply 3437

Michaelj99

7

Original post by cr7alwayz

Can anyone explain how x=4 for the last question if you remember your working out.

1)Calculate moles of sodium thiosulfate ion thing?
2) Ratio of that was 2:1 respect to I2, so times the moles by 2
3) In question it said 2 moles of Cu3+ reacts with 1 mol of I2, so times the moles of I2 by 2 (ie sodium thiosulfate moles and Cu3+ moles were the same)
4) Since the moles for Cu3+ were in 25cm3 FROM 250cm3, times it by 10 to get moles of Cu3+ in 250cm3
4) work out molar mass of whole thing, they gave u grams of 2.226? it was like 225g
5) Work out molar mass of of hydrated compounds, Cu(HCOO)2 (dot) xH20 so 153.5 (dot) x18
6) equate that to molar mass of whole thing and solve for x

Not sure about mole numbers and grams but that was my method

Reply 3438

username2640523

8

Original post by cr7alwayz

Damn, I think after finding the moles of Cu2+ I worked out the molar mass of the wholecompund by dividing the mass they gave by moles of Cu2+. Then, I took away molar mass of Cu(HCOO)2 to from that to get the molar mass of water. Then divide by 18 to get 17 for X.
So basically, I used the method on page 220 in the textbook.