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**1. Homework Statement**

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

**2. 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.

**3. 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.