# Chemistry: Converting density to molar mass given other values.

1. Sep 9, 2007

### LakeMountD

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

The density of a gaseous compound was found to be 1.23 kg/m^3 at 330K and 20,000 Pa. What is the molar mass of this compound.

2. Relevant equations

PV=nRT

3. The attempt at a solution

I don't think this is right but I assumed a volume of 1 m^3. So I used ideal gas law as follows:

n = (20,000Pa * 1m^3)/(8.314 (Pa*m^3/mol.K) * 330K) = 7.29 mol.

Then what I did was took density and multiplied it by the assumed volume of 1m^3 and got 1.23 kg/m^2. Then I took 1230g/m^3 and divided it by 7.290 mol. go get grams/mol*m^2. The answer was 168.72 but I can almost guarantee I am being retarded and overlooking something.

2. Sep 9, 2007

### rocomath

you can also use this equation

d = MP/RT

let me know what you get

3. Sep 9, 2007

### LakeMountD

I take it that is the molar volume formula?

The reason why I didn't use that is because it says in my book that the formula for molar volume (above) is used under STP conditions. Did I read that wrong?

4. Sep 9, 2007

### dextercioby

Rocophysics is right. $\rho =\frac{\mu p}{RT}$ for an ideal gas.

5. Sep 9, 2007

### LakeMountD

Okay so d = MP/RT where M is the molar mass (or molar volume?)?

Sorry I just want to make sure. And thanks for the help guys. Like I said I saw that formula in the book but I thought it said it only applied at STP.

6. Sep 9, 2007

### LakeMountD

I ended up getting ~ 153g/mol

7. Sep 9, 2007

### rocomath

that's not what i ended up getting, is this that the correct answer in the book? i'll re-do my calc.

i'm off by ~ 15

Last edited: Sep 9, 2007
8. Sep 9, 2007

### hage567

I didn't get this answer either. I got the 168.72 g/mol that was given as the answer in the first post.

9. Sep 9, 2007

### rocomath

lol i didn't even notice the answer in the first post, it's correct tho. different method but valid and good.

gj!

10. Sep 9, 2007

### LakeMountD

Wait so I did it right the first time? I don't have the answer in the book, unfortunately it is an even numbered problem.

Why didn't the formula you told me work?

11. Sep 9, 2007

### hage567

It should have. Double check your calculation.

12. Sep 9, 2007

### LakeMountD

M = d*R*T/P = 1230g/m^3 * 8.314 * 300k / 20,000 = 153.39 g/mol?

13. Sep 9, 2007

### hage567

It's 330 K, not 300 K.