A sample of xenon occupies 1.90L at 251 kPa and -18°C. What was the temperature of the xenon if the gas originally measured 1.65 L at 205 kPa?

The temperature of the xenon : 180.863 K

Combined with Boyle's law and Gay Lussac's law  

P1 = initial gas pressure (N/m² or Pa)  

V1 = initial gas volume (m³)  

P2 = final gas pressure  

V2 = finalgas  volume  

T1 = initial gas temperature (K)  

T2 = final gas temperature  

P1=251 kPa

T1=-18+273=255 K

V1=1.9 L

P2=205 kPa

V2=1.65 L

the formula mass of a molecule or a compound is the sum of the atomic weights of the atoms in its empirical formula

(that's what i know about formula mass)