# Ideal gas law and Composition

1. Feb 18, 2009

### Oblivion77

1. The problem statement, all variables and given/known data

A rigid cylinder of 1000cm cubed contains 4.168grams of an ideal gas mixture. The gas mixture consists of methane and ethane at a temperature of 100C and a pressure of 500kPa

a) Estimate the average molar mass of the mixure
b) Calculate the composition of the mixture on a mole fraction basis
c) Express the mixture composition on a mass fraction basis

2. Relevant equations

PV=nRT

3. The attempt at a solution

I calculated part a) already and found that to be 25.85kg/kmol. I am stuck on part b and c.
Thanks for any help.

2. Feb 19, 2009

### Staff: Mentor

Express mass of the sample in terms of numbers of moles of each gas.

3. Feb 19, 2009

### Oblivion77

Ok

I got mols of ethane to be 0.14 and mols of methane to be 0.26. Now what? I tried doing
0.14 / (0.14 + 0.26) to get the fraction of ethane but it is wrong.

4. Feb 19, 2009

### Staff: Mentor

Show how you calculated total number of moles of both gases.

5. Feb 19, 2009

### Oblivion77

For methane: CH4 molar mass is 16.04g/mol. We have 4.168g so M= m/n, n = m/M therefore
n = 4.168 / 16.04 = 0.259 ~ 0.26mols of methane

same thing for ethane.

6. Feb 19, 2009

### Staff: Mentor

No. You can't have 4.168g of methane plus 4.168g of ethane and 4.168g in total.

Use ideal gas equation to calculate total numebr of moles of gases.

7. Feb 19, 2009

### Oblivion77

Ok, so how would I calculate how much of each compound by just knowing the molar mass?

V/n = constant?

8. Feb 19, 2009

### Staff: Mentor

Mass is molar mass times number of moles.