Need help with chemistry

Question

How many moles of oxygen gas are required to completely combust 21.4 grams of methane gas (CH4)?

Answer 1

So Tyler L.,

You absolutely must acquire the skill to write and balance a chemical reaction.

You seem to be at sea on this process, so I'll take it slow for you.

Combustion means you produce CO2, H2O, and any other gases necessary to carry away spare atoms in your reactant molecule(s), usually as oxides.

So -- first, do you have any other atoms in your reactant (CH4) than those which appear, together with oxygen, in your products : CO2 and H2O?

You do not, therefore you should start by writing:

CH4 + O2 = CO2 + H2O . Reactant(s) are on the left, product(s) on the right.

Next step (always): pick the largest molecule in your reactants, and start tracing its atoms over to the right, one element in it at a time.

SO, you can trace 1 C over, directly. (one C in CH4, one C in CO2)

Fine.

Now, look at the H's: there are 4 on the left, but only 2 on the right.

So adjust your coefficient (that's the number in front of the molecule formula, which tells how many molecules of that are needed in the reaction) on the H2O: to make 2H2O.

You now have 4 H's on the right (2 molecules of H2O with 2 H's in each). They balance now.

Last (in this case), trace your O's BACKWARDS in the reaction: You know that you have 1x2 + 2x1 on the right, that's 4 in all: that requires a coefficient of 2 on the O2 on the left, to make it : 2O2 . Now you O's balance.

Your final balanced equation is: CH4 + 2O2 = CO2 + 2H2O

Next, figure out what types of information are given you, and what you need to produce. You are given a mass in g of CH4, but you want a moles of O2. So, reactions are always figured in moles, so convert 21.4g CH4 into moles of CH4. How do you do that? You either look up the individual masses of each of the atoms in CH4 (from the periodic table), or directly look up the molecule (molar) mass of CH4. That's 16.04 g/mole. So divide 21.4/16.04 = 1.33416458853; that's the factor by which your equation has to be "scaled up" to correspond to 21.4 g CH4. SInce you needed 2 moles of O2 in your balanced equation (above), you need to scale that up by 1.33416458853 = 2.668 .... moles. Round that to 3 sig figs -> 2.67 moles, and you're done. -- almost!

I say almost, because the purpose of this problem, Tyler, is not to obtain the particular answer. It is to acquire the skill to solve this type of problem yourself when you next encounter it (and incidentally demonstrate that you have done so). If you simply transcribe the results across without taking the time to think about them, you will fail this (and likely other) courses. Just sayin.

--Cheers, --Mr. d.