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A level chemistry

I’ve been looking through past papers and come across a really difficult question It’s from edexcel a level chemistry paper 2 2020 and it’s question 5 I think? It’s the one where you find out the values of x and yI was wondering if anyone has done it and if so how do you find the value of yTIA
Screenshot 2022-05-12 at 10.47.16.png

It is very difficult in my opinion. We know that NaOH will remove the CO2. They also make the point that all the water remains liquid. So, after burning 25cm3 of hydrocarbon, the gas volume went down by 75cm3. This must be the water as everything else is a gas. Now comes the clever part. The volume of gas decreased again after adding the NaOH and this must be due to carbon dioxide being removed. 125cm3 of gas was removed. We started with 25cm3 of hydrocarbon. The ratio between CO2 and the hydrocarbon is 125/25 which is 5:1. In other words, there are five carbon atoms in the molecule.

You could find the number of Hs by trial and error, given you know that this is a cyclic compound and you have five carbons. If you're good at maths, you can probably now see that the total decrease in volume is 125+75 = 200 cm3 . This will correspond to the volume of hydrocarbon 25cm3 and the volume of oxygen used. The volume of oxygen will be 25cm3 multipied by (x + y/4). We already know that x= 5 so we arrive at. Volume O2 = 125 + 25y/4

Adding the volumes together we obtain:

25 + 125 + 25y/4 = 200

Therefore y = 8
(edited 1 year ago)
Original post by tony_dolby
Screenshot 2022-05-12 at 10.47.16.png

It is very difficult in my opinion. We know that NaOH will remove the CO2. They also make the point that all the water remains liquid. So, after burning 25cm3 of hydrocarbon, the gas volume went down by 75cm3. This must be the water as everything else is a gas. Now comes the clever part. The volume of gas decreased again after adding the NaOH and this must be due to carbon dioxide being removed. 125cm3 of gas was removed. We started with 25cm3 of hydrocarbon. The ratio between CO2 and the hydrocarbon is 125/25 which is 5:1. In other words, there are five carbon atoms in the molecule.

You could find the number of Hs by trial and error, given you know that this is a cyclic compound and you have five carbons. If you're good at maths, you can probably now see that the total decrease in volume is 125+75 = 200 cm3 . This will correspond to the volume of hydrocarbon 25cm3 and the volume of oxygen used. The volume of oxygen will be 25cm3 multipied by (x + y/4). We already know that x= 5 so we arrive at. Volume O2 = 125 + 25y/4

Adding the volumes together we obtain:

25 + 125 + 25y/4 = 200

Therefore y = 8

took me like an hour to figure this out even after reading your reply haha, and surprisingly(or at least i think so) you don't even need the value for x, as the 25x cancels out, and you'd get 25y/4 =50 to get y=8
Is there any chance you could go into a bit more detail about how you got y I’m not really seeing it, I understand that the total volume decreased is 200cm3 just unsure about how u worked out the volume of o2
(edited 1 year ago)
Original post by Ameliaajayybrnn
Is there any chance you could go into a bit more detail about how you got y I’m not really seeing it, I understand that the total volume decreased is 200cm3 just unsure about how u worked out the volume of o2

We know that the volume of hydrocarbon is 25cm3. Let's forget about volumes and call them moles. The moles of hydrocarbon used = 25. How many moles of O2 do we need to react with it? The equation tells us that the moles of O2 needed = 25(x + y/4). If we had 10 moles of hydrocarbon, the moles of O2 needed would be 10(x + y/4) etc. etc.

In our case, we know that x=5, so we can expand the bracket to get: 125 + 25y/4.

Then we add all the volumes together to solve for y.

I wouldn't worry too much about this type of question unless you see it popping up every year. There will be easier ways to get three marks!
(edited 1 year ago)
Ohhh okay thank you so much I understand now, I wasn’t too bothered about the question coming up in the exam it was more annoying that I couldn’t get it, thank you so much for your help x

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