1. The problem statement, all variables and given/known data Given Pressure= 1 atm, Temperature=273, and Relative Humidity= 45%, find the density of air. 2. Relevant 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.