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Relative molecular mass help

I'm trying to work through stuff that was done in class while I was ill & I'm not sure I'm getting it right. Can someone have a look over it & let me know if (& where) I'm going wrong?

Q. Calculate the relative molecular mass of a vapor if 0.2g of gas occupy 400 cm3 at a temperature of 223oC and a pressure of 100KPa.

Here's what I've come up with:

PV=nRT

100 x 4x10-4 = n x 8.31 x 496
n= (100 x 4x10-4)/(8.31 x 496)
n= 9.705x10-6 mol-1

RMM= m/n

RMM= 0.2/9.705x10-6
RMM = 20607.93 g/mol-1

The reason I'm questioning this is because it just seems too large a figure

TIA
(edited 9 years ago)
I believe that the value of R should be 0.08206 rather than 8.314 because 0.08206 units are dm^3 atm/k mole which will fit with your values. The unit of 8.134 is J/k mole.


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Avatar for lovali
lovali
OP
7
Original post by laraalmahbshi
I believe that the value of R should be 0.08206 rather than 8.314 because 0.08206 units are dm^3 atm/k mole which will fit with your values. The unit of 8.134 is J/k mole.


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I've been following the worksheet we were given which states 8.31 but it does make more sense now that you've pointed out I forgot to convert the units! thanks
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ETRC
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Original post by lovali
I've been following the worksheet we were given which states 8.31 but it does make more sense now that you've pointed out I forgot to convert the units! thanks


keep r as 8.31 but you converted volume wrong.
400cm3 is not 4x10-4 m3
1m3 is 100x100x100cm3
use that to help you


also change grams to kg
Reply 4
Avatar for lovali
lovali
OP
7
Ok now I'm even more confused!

I found the original online of where the questions in our workbooks came from that has information that was omitted in ours.

Where on earth did the m= 0.27g come from?

Example 2 Calculate the relative molecular mass of a vapour if 0.2 g of gas occupy 400 cm3 at a temperature of 223°C and a pressure of 100 KPa.

P = 100 KPa = 100000 Pa
V = 400 cm3 x 10-6 = 0.0004 m3
T = 227 + 273 = 500 K
m = 0.27g = 0.27g
R = 8.31 J K-1 mol-1 = 8.31

PV = mRT/M ∴ M = mRT/PV = 0.27 x 500 x 8.31/100000 x 0.0004 = 28.04



**EDIT** Think I've figured it out to be a defective worksheet. Using the figures exactly from the question, rather than the worked example, I get:

M= 0.2x8.31x496 / 100000x0.0004 = 20.61

Am I now on the right lines? It seems that whoever made this workbook forgot to include this formula in it as all the other formulas are but PV=mRT/M isn't
(edited 9 years ago)
Reply 5
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Pigster
20
20.61 looks fine to me, so you'd have to guess it could be HF.

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