a) N2
mN = 14u
mN2 = 2*mN = 2* 14u = 28u
b) CaS
mCa = 40 u
mS = 32 u
mCaS = 72 u
c) Al2O3
mAl = 27 u
mO = 16 u
mAl2O3 = 2*mAl + 3*mO = 102 u