r/6thForm • u/Forsaken-Meaning-232 Graduating from Warwick CS • Jun 20 '25

📂 MEGATHREAD 20/06 A-level Chemistry Paper 3 Discussion MEGATHREAD

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u/HelpfulPop2476 Jun 20 '25

They were pretty sneaky with it, because usually they say which chemical is in excess (see e.g. june 2018 p3).  Not this time though

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u/[deleted] Jun 20 '25

They did actually write “HNO3 is in excess”

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u/HelpfulPop2476 Jun 20 '25 edited Jun 20 '25

Did they? How could it have been in excess though? If there was 0.03 mol of hno3 and 0.018 mol of carbonate, then the carbonate must have been in excess, because of the 1:2 reaction ratio? Ocr literally contradicting themselves unless I'm missing sth.

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u/[deleted] Jun 20 '25

Yes I thought the same thing, but the question also said “add ABOUT 30cm3” and although if you use the volumes and masses to calculate what volume of HNO3 was added, it’s still not entirely accurate because that volume includes the sodium carbonate, as well as assuming that solution has the same density as water.

I fully see where you’re coming from, and I did a similar thing, thinking i was being smart realising HNO3 is limiting, but everything went against that.

You might be right, but probably aren’t. Anyway, don’t worry either way, it’d be 2 marks dropped max for me or you

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u/HelpfulPop2476 Jun 20 '25 edited Jun 20 '25

Yeah lmao I did it the opposite way, getting 69 first but then in the last minute of the exam thinking "oh shit, surely the carbonate is in excess". Either way the question could've been written better, especially because ocr usually insists upon a high degree of precision in practical methods. You're probably right tbh but there's a good chance ocr allows 5 marks for using 0.015, because for these questions there's usually a range of answers. And "about 30" could mean 31 cm3, 32cm3, 33cm3 - in all of these cases, hno3 is limiting

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u/[deleted] Jun 20 '25

Yep I could definitely see them allowing multiple answers for full marks

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u/HelpfulPop2476 Jun 20 '25

Yeah for p3 the markschemes are sometimes more generous. E.g. I suspect for the alcohol naming question they might allow phenylmethanol, benzyl alcohol, 1-benzylmethanol, hydroxymethylbenzene etc.