You can use the Clausius-Clapeyron equation to calculate saturation vapour pressure (es). You can then calculate mixing ratio ws=0.622(sat vap./(psfc - sat vap.)) you can find these equations by googling.
Relative humidity is: (Q2/ws)*100
Another method is to also compute vapour pressure, e, and RH= (e/es)*100
Make sure you use the correct units. es is returned in hPa.