# Detemining the mass of water vapour in air (Ideal gas law)

1. Mar 8, 2012

### Zewz

1. I am supposed to calculate the mass of water vapour in one cubic meter of air that has a relative humidity of 80% at 20°C.

2. p[v] = (eM[w]) / (RT)

Where:
p[v] = vapour density
e = water vapour pressure
M[w] = molar mass of water
R = gas constant (8,314 J / (mol*K)
T = temperature (293,15°K)

3. I am struggling with in what unit the vapour density should be measured, as well as the pressure, Pascal? I know this problem is embarrassingly easy, but I've been trying to get the answer for several days, reading up on it online... I miss my old physics-book!

Help would be sincerely appreciated!

2. Mar 8, 2012

### Camille

Probably it's best if you use SI units. You can take $ρ = \frac{m}{V}$ in $\frac{kg}{m^3}$ and pressure in Pascals.