# Chemo Reacting Volumes Of Gasses q's

Hi guys i need a bit of help, with a couple of questions, if possible can someone show me how to work these with the answer, thanks a lot, any help appreciated!

1.A mixture of 2 litres of methane (Ch,4) and 4 litres of oxygen was ignited causing a combustion. Calulate the composition and volume of the gaseous mixture remaining.

2. A mixture of 40cm^3 of hydrogen and 40cm^3 of oxygen was sparked to create an explostion. calculate the compostition and final volume of the resultant gas mixture.

any help guys!
cheers

si ps, sorry i only just found there was a hwk zone bit, if a mod wants 2 move it is fair enough

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Monique
Staff Emeritus
Gold Member
Hi SiCharlton, how are you every supposed to be able to solve this?? Is the system enclosed? What are the pressures? How much heat is released? I can't help you on this one..

It's just like any other of these chemistry problems. First, figure out how many moles of material you have. Then figure out the limiting reagent. Then figure out how much product you get. The total volume after reaction will depend on the number of mole of product plus the number of moles left of the limiting reagent.

You do know how to get moles from volume of gas and vice versa, right?

chemicalsuperfreak thankyou very much helped a lot!!!
:>:>:> all done now,
cheers
si

Monique
Staff Emeritus
Gold Member
Originally posted by Chemicalsuperfreak
It's just like any other of these chemistry problems. First, figure out how many moles of material you have. Then figure out the limiting reagent. Then figure out how much product you get. The total volume after reaction will depend on the number of mole of product plus the number of moles left of the limiting reagent.

You do know how to get moles from volume of gas and vice versa, right?
Well, yeah, but..

The volume of a gas depends on its temperature and under how much pressure it is.. there is nothing said about that in the question, especially.. what happens after an explosion? The volume would fill the room.

Anyway, if it solved it for you, SiCharlton, than it is all good.

Originally posted by Monique
Well, yeah, but..

The volume of a gas depends on its temperature and under how much pressure it is.. there is nothing said about that in the question, especially.. what happens after an explosion? The volume would fill the room.

Anyway, if it solved it for you, SiCharlton, than it is all good.
If it's not stated assume STP. You can also assume it's a sealed container, a closed system, and that the final state will also be STP. Just to simplify things. You also have to assume a complete reaction resulting in a single prouct. And the latter is probable the most difficult to justify if this were a real world problem.

Bystander
Homework Helper
Gold Member
STP is not a necessary assumption; it is necessary to assume ideal gas behavior, and that reactants and products are at the same conditions of T,P. Information enough is given to determine mole ratios only.

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Originally posted by Bystander
STP is not a necessary assumption; it is necessary to assume ideal gas behavior, and that reactants and products are at the same conditions of T,P. Information enough is given to determine mole ratios only.
Yes, for this question. But often there are questions where the pressure and temperature do matter, and it is not given. And in these cases STP is conventionally assumed. Plus, knowing the volume of one mole of gas at STP makes for quite an easy "back of the envelope" approach to this problem.

Monique
Staff Emeritus