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# in need of a formula.. watch

1. "Gas F has 82.8% carbon and 17.2% hydrogen. 0.290g of the gas occupies 120.0cm3 at rtp"

I've worked out that the empirical formula is C2H5.

How do I work out the Molecular Formula?
2. n = V/Vm = 0.12 liters / 22.4 mole/liters
n = m/M, M = m/n = 0.29 g /0.005 mole = 58 g/mole

Molar mass of empirical formula = 29

0.828*29 = 24.012 g/mole (carbon)
0.172*29 = 4.988 (hydrogen)

24.012 / 29 = x / 58

29x = 24.012*58, x = 48.024 ~ 48 g /mole - molar mass of C in the molecular formula

4.988 / 29 = y / 58

29y = 4.988*58, y = 9,976 ~ 10 g/mole - molar mass of H in the molecular formula

48/12 = 4
10/1 = 10

So the molecular formula is then C4H10

Sorry for not properly writing it all down(all the units and stuff), but I haven't done chemistry in over 4 years. In fact, I am amazed I still remember this stuff. Oops, sorry, at first wrote the molar mass of the empirical as 27, not 29, fixed.
3. (Original post by Neotinea)
"Gas F has 82.8% carbon and 17.2% hydrogen. 0.290g of the gas occupies 120.0cm3 at rtp"

I've worked out that the empirical formula is C2H5.

How do I work out the Molecular Formula?
One mol of a gas has a volume of (approximately) 24 dm^3 at RTP
You have the mass for 120 cm^3
get the mass for 2400cm^3 (Which is 24 dm^3)
and that's the mass for one mol.
You now have the molecular mass and the empirical formula.
Use it to work out the molecular formula.
4. thank you both!

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