Answer
HFC-134a ($CH_2FCF_3$) = $102.03$ grams
HCFC-124 ($CHClFCF_3$) = $136.47$ grams
Work Step by Step
HFC-134a $(CH_2FCF_3) = 2*12.011+2*1.008+4*18.998=102.03$ grams
HCFC-124 $(CHClFCF_3) = 2*12.011+1*1.008+4*18.998+1*35.45=136.47$ grams