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!