[Oblivion77]

A rigid tank having a volume of 1m cubed contains a mixture of 1.8 kg of nitrogen and 2.9 kg of an unknown gas. The mixture has a pressure of 290kPa and a temperature of 360K. Determine.

a) The average molar mass of the gas mixture
b) The molar mass of the unknown gas
c) The composition of the mixture on a volumetric basis

I already did part a) and got 47.47kg/kmol. I am stuck on parts b) and c). Thanks

[ARGOS++]

Dear Oblivion77;

For a.) I got a value 2.5% off from your result.
Hint for b.):  How many moles are 1800g Nitrogen?; - and what is your n_total from a.)?

With this you should be able to calculate b.) and c.).
Good Luck!
                    ARGOS⁺⁺

[Oblivion77]

The mass of the unknown gas is actually 2.8kg

I still can't seem to get the answer.

1800g of Nitrogen. 14.01g/mol so we have 128.48mol of nitrogen. (Could also be 64.34mol if its N2, wasn't sure which one)

4600g total which has a molar mass of 47.47g/mol, so there is 96.9mol

What am I doing wrong? And what should I do next?

[typhoon2028]

It should be assumed that nitrogen is diatomic.  Its molar mass is 28 g/mole.

I calculate a) = 52.104 g/mole

b) 111.89 g/mole

c) 71.27% Nitrogen, 28.73% unknown 


I used the ideal gas law to calculate total number of moles first
PV=nRT, solve for n

you know 1,800 g of the tank is filled with nitrogen.

I solved c) using partial pressures.

[Oblivion77]

It should be assumed that nitrogen is diatomic.  Its molar mass is 28 g/mole.

I calculate a) = 52.104 g/mole

b) 111.89 g/mole

c) 71.27% Nitrogen, 28.73% unknown 


I used the ideal gas law to calculate total number of moles first
PV=nRT, solve for n

you know 1,800 g of the tank is filled with nitrogen.

I solved c) using partial pressures.

Thanks, but unfortunately those are incorrect. I have the answers to the problem but can't seem to get part b or c

a) 47.47g/mol (I got this one)
b) 85g/mol
c) 66% and 34%

[typhoon2028]

Yes, sorry I used 270 kpa in my calculations, instead of 290 kpa.

However, I still do not get your values. 

Plugging in 290 kpa I get:

Total tank moles = 96.89 moles
Moles of N2 = 64.3
Unknown moles = 96.89 - 64.3 = 32.6 unknown moles
Partial Pressure N2 = 192.3 kPa
Partial Pressure Unknown = 97.7 kPa


a)  (1800 + 2900) / 96.89 = 47.8 g/mole
b) 2900 / 32.6 = 88.9 g/mole
C)66.3% N2, 33.7% unknown

[ARGOS++]

Dear typhoon2028;

Your values are correct expect the one for b.).

You forgot Oblivion77’ correction for the weight of the unknown gas.
So if you use 2.8kg for the weight (for a. & b.) then you result also in 85.89g/mole and that’s correct.

typhoon2028: Would you please next time not explicitly solve others homework, because it’s against the forum rules. But you are already a big help if you give hints to push them in the right direction.

Good Luck!
                    ARGOS⁺⁺