Find Air Density: Pressure=1atm, T=273, RH=45%

[physicsss]

Homework Statement

Given Pressure= 1 atm, Temperature=273, and Relative Humidity= 45%, find the density of air.

Homework Equations

1. PV= nRT

2. density of air= M*P/(R*T), where M is the molecular weight of air.

3. M= 18*Y_H2O +M_dry_air *(1 - Y_H2O),
where Y_H2O is the mole fraction of H2O, and M_dry_air is the molecular weight of dry air, which is given as 29.95g/mol.

The Attempt at a Solution

I'm having trouble finding out Y_H2O, with which I can use the third equation to find the molecular weight of air, then use equation 2 to find the density of air. I know to find Y_H2O, I need to use the relative humidity relationship. Can someone tell me what relative humidity is and how I can use it to solve the problem? Thanks.

 

[GCT]

http://en.wikipedia.org/wiki/Relative_humidity

 

[GCT]

Use the relative humidity equation to find the partial pressure that is due to the gaseous air, then go on from their to find the density of air using the approach you've outlined in the original post.