### QUICK GASEOUS MOLAR CALCULATIONS | Online A Level Chemistry Tutor

Online A level chemistry tutor, Paul Morgan, explains how to calculate volumes for a gaseous chemical reaction using the online whiteboard bitpaper to Atul Rana, fellow online tutor.
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Atul
Hi, I'm Atul and I'm in London right now.
Paul
Hi, I'm Paul and I'm in Burnley at the moment.
Okay. So we're going to look at a gas calculations using balanced equations.
The good thing about gas calculations is that we can just use the volumes of the gases.
We don't have to calculate the moles because one mole of any gas is 24 decimetre cubed.
And so we can just use the proportions of the volumes of gases to do the calculation anyway. We'll have a look at this.
So in our example 30 centimetres cubed of Xenon react with 20 centimetres cubed of Fluorine.
The gases react according to the equation assuming the temperature and pressure remain constant.
So it's asking for the final volume of all the gases after the reaction is complete.
So what I would always say with any calculation is write it under the balanced equation.
Atul: (01:22)
Okay.
Paul: (01:27)
So I'll place the equation over here and I use what's called the ICE method. Now, most chemistry students are familiar with the ICE method from equilibria but you can use it just for normal reactions. And I'll show you how.
So the I in ICE stands for initial, so we're looking at the initial volumes of gases.
So we've got 30 centimetres cubed of Xenon, 20 centimetres cubed of Fluorine. And initially we've got nothing of Xenon Flouride.
So then we look at the C in ice, which is the change. So we're looking at looking at how much of the reactant is used up and how much of the product is formed.
Paul: (02:41)
So here because we've got less of the fluorine and it's a one to one ratio, then that's going to be the limiting reagent.
So that's how much xenon and fluorine are going to react. So all of the fluorine will react.
The 30 centimetres cubed Xenon is in excess because it's a one to one molar ratio with 20 centimeters cubed Fluorine.
So the E stand usually stands for what we'd got left at equilibrium. That this isn't an equilibrium, it's a reaction. So I'll just put here, this is what we've got left at the End of the reaction instead.
So you can say if we started with 30 centimeters cubed Xenon and 20 reacted, we must be left with 10 centimetres cubed of Xenon.
All the Flourine reacted so we are left with 0 centimetres cubed of Fluorine.
And we formed 20 centimetres cubed of Xenon Flouride.
So our total gas volume at the end is 30 centimetres cubed.
And we see that that is option C.
So for gas calculations from balanced equations, we can just (we couldn't do this with solutions or solids) use the centimeters cubed we don't have to convert them into moles when using gases.
Atul: (04:36)
Great. That sounds great. So I suppose if you wanted to, you could still converted it into moled and do the method the student is familiar with already if they are a little bit unsure of what to do.
Paul: (04:54)
Exactly. Yes. They could use that 1 Mole of any gas is 24,000 centimeters cubed and convert all these values into moles, and then at the end, convert them back into centimeters cubed.
And they get the same answer but it would be a lot longer method of doing it.
Atul (05:20)
Great. And also in this case, you have to figure out which one is the excess but that's fairly straightforward since you always get 24,000 centimetres cubed for one mole.
Paul: (05:35)
Yeah. You can just go off volumes to figure out excess because it's a one to one ratio.
When it's a one to one ratio, the limiting reagent is the lower moles, or in this case, the lower volume.
Atul: (05:58)
Sounds good. Yeah. I look forward to the next video, whatever that might be.
Paul: (06:04)
Okay. Thanks. Bye.
Atul:
Bye. Thanks. Bye.
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Paul Morgan
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