OCR Unifying Concepts in Chemistry...

Well, just came home from doing that exam. How did everyone else find it? I found it quite hard to be honest with u. I found some of the past papers a piece of piss, but this one was just strange. For some of the questions I just didn't know what to do!

What is the formula for Calcium Phosphate? I put CaPO4 but I really don't have a clue. Hmmmm... que sera sera I guess! I just couldn't get to grips with certain questions. Oh well!

Opinions?

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Reply 1

allodoxa

Yes that question annoyed me.

The first part was ok, (P4O10, right??), but I couldn't do the rest of it.

And there was one really easy question I screwed up for no aparant reason.

But apart from that... it was ok, hopefully... unless I made any stupid mistakes.

Reply 2

annaholden

i just got back too, thought it was harder than the past papers aswel, and didnt get that q either, i either put CaPO3 or CaPO4, cant remember! what did u get for the volumes on the last one? i got 0.06dm and 0.12dm
oo, and the one where u had 2 work out the ph? i got 3 i think, and then 12.4 or sumut like that for the 2nd one. hmmmm hope it was rite! , oh, adn there was one where u had 2 work out the concentration of o2 gas? i put 0.3 mol dm but i kept getting different answers wen i did it, what did u get?

Reply 3

annaholden

yey, i got p4O10 aswel!

Reply 4

ThunderCat8

Ya, P4O10 I got. I also got 0.3 for the oxygen one. The very first question was odd too. Initial Rate of reaction...um, the rate at the very beginning?!?

I didn't like the last part to the first question asking what the conc of the stuff would be after 3 seconds. I just couldn't do that for some reason. I put that HCl would stay the same conc as it is a catalyst, but I really just bullshitted it!

Hmmm, not too bad an exam, but not particularly nice either.

Reply 5

allodoxa

Yeah the HCL one it right. The other one, the conc should have halved because the half life of the reaction was 3 seconds. I think.

I'm a bit annoyed with myself for messing up those two questions :mad:

Never mind. I think (hope) biochemistry and trends and patterns will be a bit easier, and I've got all weekend plus monday to revise for them.

Reply 6

Willa

well i'll put my two cent in now before people start discussing the answers, cos i really dont wanna know what the answers were - as far as i thought, that exam went pretty well. It wasnt a piece of piss, but i felt there wasnt much that really stretched me. Of course, i may will be clueless about alot of the questions, and thought i knew the answers when i didnt - we'll see. But my feelings after that exam are quite good - i think i should still be able to get an A overall even if i screw up the remaining two papers

Reply 7

ThunderCat8

allodoxa

Yeah the HCL one it right. The other one, the conc should have halved because the half life of the reaction was 3 seconds. I think.

Ah, I thought that was right, but I only thought of it at the end as the invigilator said Pens Down. So I won't get that mark, oh well! I hate exams!

Reply 8

humanpunk

I thought it was a good exam, in general, although some questions were a little akward...the practical makes up the other half of the module though and I had a bad day with that :s-smilie:

p.s. I thought that as the acid was H3PO4, then PO4 must have a 3- charge, and as Ca has a 2+ charge I figured the formula was Ca3(PO4)2 ? It made all the equations balance really easily anyway so i was happy :rolleyes:

Reply 9

madmazda86

*looks bemused* That was all a bit of a blur, really - I'm now in the post-exam zombie stage :eek:

I made a complete pigsear of that Oxide A identification - I was rambling on frantically about oxidation states because I couldn't remember the phosphorus oxidation formula and totally didn't notice the empirical formula calculation at the top until five minutes before the end :frown:

And even when I did the calculation and corrected my answer I was so sure it would be PO4 that my brain overrode all reason upon seeing O = 2.5 and multiplied it to get 4, while totally ignoring the formula mass. Gah, I can be such a prat when it comes to empirical calculations AND not reading the question properly.

Didn't occur to me about the HCl being a catalyst and therefore its concentration would remain the same. Meh :frown:

Oh well, the rest of it wasn't too bad and I'm just glad that I didn't have to remember any pH ranges for indicators as they were all nicely laid out in front of me!

Reply 10

annaholden

humanpunk

yeah, that sounds rite! i got into a rite mess wit mine! ah wel, it was only 2 marks so not too bad!

Reply 11

madmazda86

Calcium phosphate = CaHPO4 - as it was reacting with H2SO4 the extra H had to come from somewhere. Sadly I thought of this in the exam but dismissed it and put CaPO4 instead. I've given up on that question as a total loss so 0/7 for that!

Reply 12

Willa

it cant be CaHPO4 because that's calcium hydorgen phosphate. I put the same answer as the person above - Ca2(PO4)3

Reply 13

madmazda86

Then where does the extra hydrogen atom come from? You can't get it out of Ca2(PO4)3...

CaHPO4 + H2SO4 ---> H3PO4 + CaSO4

Reply 14

cobra

It isnt CaHPO4 it is Ca3(PO4)2 + 3H2SO4 --> 2H3PO4 + 3CaSO4

Reply 15

cobra

Overall i found this paper strange and i expected at least a mention of buffers, but it was easy all through

Reply 16

mongoose

Well i now know i've lost seven stinking marks for that question! I did initially work out the molecular form. to be P:O - 4:10, but then in a moment of inexplicable madness i changed me answer to 6:14 :mad:

..... what did you guys get for the very last question, for the volume of Base and Acid needed?

Reply 17

cobra

mongoose

..... what did you guys get for the very last question, for the volume of Base and Acid needed?

60cm3 and 120cm3

Reply 18

mongoose

cobra01977

60cm3 and 120cm3

Well at least i got those three marks! Do you know if the examiners mark negatively for woffling (even if the eventual answer is correct)? How about the for the last question of the very first section, a three-mark question asking for the concentration of the reactants after 3secs? Oh, and what indicator did you choose for the titration? I put the second.

Reply 19

madmazda86

cobra01977

It isnt CaHPO4 it is Ca3(PO4)2 + 3H2SO4 --> 2H3PO4 + 3CaSO4

Ahhhh - I bow to you! :wink:

Sorry if I worried people with that - I am rather hopeless at balancing equations and I did make a total mess of that question anyhow. I just typed Calcium phosphate into Google and that was what came up.