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# Calculations with volumes? Help (Avogadro's law)... watch

1. So my chemistry teacher 'taught' us the formula relating volume, moles and 24dm^3, and then gave us a sheet with calculation questions for homework. Now I am sat here trying to work this out, because we did nothing like it in the lesson, she acted although all we had to do was multiply by or divide by 24? Could someone please help me out and explain how to do the following question? (I've changed the numbers so it's not cheating (Or..whatever), I just need to know the method)

So I would be grateful if someone could help?

Carbon burs in air to produce carbon dioxide gas as follows:

C + O2 --> CO2

i) What volume of carbon dioxide is given off when 8g of carbon burns completely in air?

ii) What mass of carbon is needed to produce 600cm^3 of carbon dioxide?

This could be super duper easy but I wouldn't know it, I've been searching for some kind of clear explanation for a while now... Maybe I'm just dumb?

If you don't feel like answering, it would even be helpful if you could post a link with some kind of explanation?
Anyway, thanks
2. Ok so I'll try to explain because it's not that difficult at all, but I know how big of a disadvantage a bad teacher can be...

So you need to remember that Volume= number of moles (n) x 24 for dm^3 or 24000 for cm^3 --> This depends on what the question is asking. If it doesn't tell you explicitly what the volume should be in just multiply number of moles by 24 to get the volume in dm^3. Here's the formula triangle

For the first part of the question you don't know the number of moles, so you use this formula to work it out. n=mass/Mr -> 8/12 = 0.667...

Now you want to find the volume so you do 0.667 x 24 =16dm^3. Or 16000cm^3. (n x 24000)

The second question requires you to work backwards: n= volume/24000 (this time you HAVE to divide by 24000 because the volume is given in cm^3). So n=0.0250 mol. You then use the second triangle again-> mass = 0.0250 x 12= 0.3g.

I don't know if this makes much sense - be sure to let me know if something doesn't! You'll become better at this with practice, look through past papers for similar questions and do them over and over until you're 100% confident you can answer any question on gas volumes Good luck.
3. Okay, thanks for that! I managed to get some answers by just messing about with the numbers given, but obviously I need to actually understand it... It was mainly the second part of the questions I was confused with, because I hadn't realized you should just use 24000, so I was converting the numbers in and out of dm^3 and using the formula.

I will certainly have to learn a lot chemistry unit 3 at home, but it should be okay with practice and past papers... My chemistry teacher somehow thinks it's teaching if she gives us a topic and then says "Research a certain area of the topic, and then teach the rest of your group" -_- Anyway, thanks a lot for the explanation, it all seems easy now!

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Updated: January 26, 2015
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