The French chemists, Pierre L. Dulong and Alexis T. Petit, noted in 1819 that the molar heat capacity of many solids at ordinary temperatures is proportional to the number of atoms per formula unit of the solid. They quantified their observations in what is known as Dulong and Petit's rule, which says that the molar heat capacity, C P , of a solid can be expressed as C P = N ⋅ 3 R where N is the number of atoms per formula unit and R is the universal gas constant. The observed heat capacity per gram of a compound containing rubidium and oxygen is 0.64 J ⋅ K − 1 ⋅ g − 1 . Use Dulong and Petit's rule to determine the empirical formula of the compound.