A method hydrofluorinates an olefin of the formula: RCX=CYZ to produce a hydrofluoroalkane of formula RCXFCHYZ or RCXHCFYZ, where X, Y, and Z are independently the same or different and are selected from the group consisting of H, F, Cl, Br, and C1-C6 alkyl which is partially or fully substituted with chloro or fluoro or bromo; and R is a C1-C6 alkyl which is unsubstituted or substituted with chloro or fluoro or bromo. The method includes reacting the olefin with HF in the vapor phase, in the presence of SbF5, at a temperature ranging from about −30° C. to about 65° C. and compositions formed by the process.