# Chemistry A level- gas/mole calc

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#1
1 molcule of a gas has mass 2.658x10-23g.
What is poss formula of gas?

1) CH4
2)O2
3)SO2
4)SO3

-now i thought bc n=mass/mr and n here is avogadro's constant
you'd do 2.658x10^-23/ 6.02x10^23 but that obv doesn't give the right answer.
-ik i'll get 16 so CH4 (the right answer) if i mutiply mass and avogadro's but wondering what the theory/maths/reasoning is behind it?
0
4 months ago
#2
(Original post by :)Esss:))
1 molcule of a gas has mass 2.658x10-23g.
What is poss formula of gas?

1) CH4
2)O2
3)SO2
4)SO3

-now i thought bc n=mass/mr and n here is avogadro's constant
you'd do 2.658x10^-23/ 6.02x10^23 but that obv doesn't give the right answer.
-ik i'll get 16 so CH4 (the right answer) if i mutiply mass and avogadro's but wondering what the theory/maths/reasoning is behind it?
1 mole of molecules is equal to the mass in grams of the relative formula mass of that molecule. So for example carbon has a RFM of 12 and one mole of carbon atoms weigh 12g. So in this case we multiple the mass of one molecule by a mole to give us the RFM, does that help. If not I will attach a YouTube video with a good explanation of moles :https://www.youtube.com/watch?v=Z_TjGRPPR9Q
0
4 months ago
#3
(Original post by :)Esss:))
1 molcule of a gas has mass 2.658x10-23g.
What is poss formula of gas?

1) CH4
2)O2
3)SO2
4)SO3

-now i thought bc n=mass/mr and n here is avogadro's constant
you'd do 2.658x10^-23/ 6.02x10^23 but that obv doesn't give the right answer.
-ik i'll get 16 so CH4 (the right answer) if i mutiply mass and avogadro's but wondering what the theory/maths/reasoning is behind it?
You're correct that n = Mass / Mr, but n isn't Avogadro's constant, it's the number of moles.

So in order to use it, you'd need to set n to 1/(6.02x10^23) (i.e. just one molecule).
Then you could say that 1/(6.02x10^23) = 2.658 x 10^-23 / Mr
Therefore Mr = 2.658 x 10^-23 / (1/(6.02x10^23)) = 16.00116

But that seems to be really over complicating things if you ask me! (Just showing you how you could use the formula you quoted if you wanted to).

By far the easier solution would be to say I know 1 mole contains 6.02 x 10^23 molecules. So the mass of one mole (which is what Mr is!) will be 6.02 x 10^23 x the mass you've been given. So you multiply them and get the same answer.
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