OCR Unifying Concepts in Chemistry...

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cobra
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#21
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#21
(Original post by mongoose)
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.
i got thymol blue and for the conc of those reactants after 3 sec i did (initial conc) - (initial rate x 3), i think thats wrong but i only need 49% for an A. They dont mark negativly but it might cause then to miss some of the marks you should have got and you migh loose your mark for quality of comunication
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Flicker
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#22
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#22
i was pleased with how it went although i've thought of a few marks i missed...it felt like an odd paper, i kept checking there wasn't a questoin 5 - where were the buffers??? but apart from that it was good
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ThunderCat8
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#23
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#23
I said to use Brilliant Yellow as the indicator as I worked out the end-point ot be at pH 7.7. Hmmm...

Wish there had been a buffer question as I think buffers are easy. Damn you OCR! Damn you!
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cobra
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#24
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#24
(Original post by ThunderCat8)
I said to use Brilliant Yellow as the indicator as I worked out the end-point ot be at pH 7.7. Hmmm...

Wish there had been a buffer question as I think buffers are easy. Damn you OCR! Damn you!
i didnt calculate it but i thought stong base/ weak acid end point is always a bit above 7 so i chose the one with the appropriate range, it fits with the surve i drew anyway
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ThunderCat8
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#25
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#25
The curve confused me somewhat as the question before asked to work out the pH of 0.02moldm-3 KOH but the titration was using 0.01moldm-3 KOH which made the pH for that 12. Starting point was pH of Ethanoic acid which was 3.4 I think. So mid point is about 7.7 or so...it was strange question, i'm not sure why the concentration of the KOH was different.
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Emzeebubs
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#26
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#26
yeh, i chose the thymol blue or whatever it was. also, i wrote Ca3(PO4)2 but thought it was wrong so didnt bother writing the equation
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ResidentEvil
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#27
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#27
(Original post by cobra01977)
It isnt CaHPO4 it is Ca3(PO4)2 + 3H2SO4 --> 2H3PO4 + 3CaSO4
correct
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cobra
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#28
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#28
(Original post by Emzeebubs)
yeh, i chose the thymol blue or whatever it was. also, i wrote Ca3(PO4)2 but thought it was wrong so didnt bother writing the equation
You didnt bother, you should always try, look what happened this time, just dont allow ur self to be confused by anything.

Oh and by the way thanks to who ever it was who bowed to me and massaged my alredy bulging, after that chem exam, ego
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ResidentEvil
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#29
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#29
(Original post by ThunderCat8)
I said to use Brilliant Yellow as the indicator as I worked out the end-point ot be at pH 7.7. Hmmm...

Wish there had been a buffer question as I think buffers are easy. Damn you OCR! Damn you!
brilliant yellow, correct
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ResidentEvil
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#30
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#30
I thought it was a nice paper overall, and it's probably one of the less hard ones in Unifying Concepts. Some of the past paper questions are alot harder. We were kinda given a break. And dont forget, only four questions :eek: that surprised me.

U should have set up a poll
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annaholden
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#31
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#31
i did the brillian yellow aswel as far as i can remember, coz the midpoint looked around pH 7ish
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ResidentEvil
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#32
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it's brilliant yellow for definite, coz all the intelligent people in my class got that, and my chemistry teacher looked at it, and thought similar. But perhaps it depends on how u draw your graph? If ur graph doesnt really meet the vertical section of yer chosen colour then they'll penalise u? Oh what end point did u all get?
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cobra
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#33
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#33
(Original post by ResidentEvil)
brilliant yellow, correct
i think its debatable depending on the curve you drew and if you actually calculated the end point
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Stunt-101
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#34
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#34
Can anyone confirm these:

Q1/ HCL and maltose were both FIRST order
When the asked about half-lives it was half the maltose concentration but the same concentration of HCL as it was a catalyst.

0.3 was the concentration of oxygen in the question about Kc

Last Question:
The oxide A was P4 O10, it was CaP2O4 or something, but i remember making the phosphate a overall 2- ion.

30cm3 for HPO3 and 60 cm3 for NaOH

Oh yeah I got brilliant yellow but did a very scruffy graph

Thats all I remember for now, Thanx
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ShadowStorm
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#35
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#35
(Original post by ResidentEvil)
It's brilliant yellow for definite
I wouldn't go blahing all over that it's brilliant yellow, because I'm fairly certain you might be wrong. Weak acid + Strong base means use something with a range above 7. You follow where the change in pH is fastest. I can't remember the range for brilliant yellow, but thymol blue seemed the most logical choice because of this fact.
The end point of the reaction is irrelevant - neutralisation always occurs at ph 7. You want to find the point where change is fastest.
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giddychick
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#36
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#36
i think i got 120cm3 for the NaOH n 6cm3 for the other thing whatever it was in the very last question!
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cobra
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#37
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#37
(Original post by Stunt-101)
Can anyone confirm these:

Q1/ HCL and maltose were both FIRST order
When the asked about half-lives it was half the maltose concentration but the same concentration of HCL as it was a catalyst.

0.3 was the concentration of oxygen in the question about Kc

Last Question:
The oxide A was P4 O10, it was CaP2O4 or something, but i remember making the phosphate a overall 2- ion.

30cm3 for HPO3 and 60 cm3 for NaOH

Oh yeah I got brilliant yellow but did a very scruffy graph

Thats all I remember for now, Thanx
Q1 sounds right, 0.3 sounds right, the oxide is right but it was Ca3(PO4)2, and i and somebody else on here got 60 and 120
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Emzeebubs
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#38
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#38
well i sed thymol blue cus it was a weak acid and strong base therefore thought that the equivalence point wud b greater than pH 7....grr, another one to add to the "wrong" list
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Stunt-101
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#39
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#39
(Original post by madmazda86)
Then where does the extra hydrogen atom come from? You can't get it out of Ca2(PO4)3...

CaHPO4 + H2SO4 ---> H3PO4 + CaSO4

Sulphuric acid catalyst i thought in the exam, now i know im wrong
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cobra
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#40
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#40
(Original post by ShadowStorm)
I wouldn't go blahing all over that it's brilliant yellow, because I'm fairly certain you might be wrong. Weak acid + Strong base means use something with a range above 7. You follow where the change in pH is fastest. I can't remember the range for brilliant yellow, but thymol blue seemed the most logical choice because of this fact.
The end point of the reaction is irrelevant - neutralisation always occurs at ph 7. You want to find the point where change is fastest.
I agre with your reasoning, that is what i thought and did.
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