what is wrong with my reasoning? ( Chem hw)

1. Which is more dense, humid air or dry air? Assume the temperature and pressure are the same. Explain your reasoning (2 pts)

PV = nRT. Density = n/V = (mass/MolarMass) / V
Since T and P = Constant... and we can assume we have equal volumes and equal masses of both humid and dry air...

For Dry Air: [More air molecules(O2 and N2) than H2O molecules]
Density = (mass/MolarMass) / 1L

Assumed Mass of 1 and Volume of 1:
P = 1g / 28g/mol (N2) or 32g/mol (O2)

For Humid Air: [More H2O molecules than molecules(O2 and N2) ]
Density = (mass/MolarMass) / 1L

Assumed Mass of 1 and Volume of 1:
P = 1g / 18g/mol (H2O)

I know correct answer is Humid Air = less dense. However, the above math shows that humid air is more dense...Whats wrong?

Intuitively, only difference between humid and dry air is some molecules of humid air are molecules of N2 and O2 instead of H2O.

Since H2O molecules 18amu < 32 or 28amu of O2 or N2, Humid air is lighter. Keeping mols, temp, atm, volume constant for both cases, since humid air = lighter, its less dense.