Simple Mole Conversions

jacksonpeeble

What is the mass of methane (CH₄) if it occupies 32.3 L at STP?

We got a whole bunch of these problems on a worksheet with minimal instruction. If somebody could please aid me in solving this one, then I should be able to solve the rest.

I assume that I must first find the masses of the individual components...

C = 12.0107 * 1 = 12.0107
+H = 1.00794 * 4 = 4.03176
---------------------------
16.0483

Then I honestly don't know what to do... I don't want the answer; I'd just really appreciate if somebody could show me how to do this. I tried searching for a guide online, but none of them really had what I was looking for.

Thank you!

symbolipoint

STP means Standard Temperature and Pressure.
Use PV=nRT.

Borek

Which - sadly - doesn't mean anything.

jacksonpeeble: you have to know what temperature and pressure your teacher want's you to use as STP, as this thing is not precisly defined.

jacksonpeeble

1 mole = 22.4 L at STP according to my teacher.

What do I do with that equation? I assume that it is the same as (Pressure x Volume)/(Temperature x Moles) = R, but how is this applied?

Thanks for the help so far, though!

fayeshin

and then let 32.3 L = x mol at STP. you can get x. do you know how to do now? (forget that equation...for this problem, it is useless.)

Borek

How many moles of methane then?

What is molar mass of methane?

While PV=NRT is the general method, that will give exactly the same results if applied correctly, you can solve the question using just the information quoted above.

jacksonpeeble

Ah, so it's essentially just a ratio?

So it would be 32.3/22.4 = 1.44 Moles?

Borek

Now moles to mass and you are ready.

Note, that if you will be given exact pressure and/or temperature value you will have to use PV=nRT equation.