# Homework Help: Molar mass

1. Mar 20, 2006

### mohlam12

Hey everyone,
I have this problem to solve:

A hydrocarbon (CxHy) contains 92.3% of Carbon and 7.7% of Hydrogen.
1) What is its molar mass, if the density of its smoke to the air is d=2.69 ?

92.3/7.7 = 12
So 12x=y
This is the first equation. I have to find a second one to find what is x and what is y, in order to find the molar mass of CxHy.

Is the density of water to the air can be written as d=(molar mass of the compound)/(molar mass of the water) ???

Anyhelp/hints would be appreciated.
Mohammed

2. Mar 20, 2006

### PPonte

Which are the units of this density, g/dm^3 ?

3. Mar 20, 2006

### Staff: Mentor

Sounds like it is ratio of its density (in gaseous form) to the air density.

"Molar mass" of the air is about 0.78*28+0.21*32+0.01*40 (ie air has density as if it was gas of this molar mass). Hydrocarbon molar mass is 2.69 times higher.

4. Mar 21, 2006

### mohlam12

Got It!! Thanks Everyone :)

Last edited: Mar 21, 2006