Definitions of molecular mass, molecular weight, molar mass and molar weight. • NaBH4 + CH2O + H2O = CH4O + NaOH + BH3 :: Chemistry Applications:: Borane BH3 Molar Mass, Molecular Weight. Borane-tetrahydrofuran complex. (1 u is equal to 1/12 the mass of one atom of carbon-12) Borane tetrahydrofuran coMplex. 14044-65-6. BH3.THF Molecular mass (molecular weight) is the mass of one molecule of a substance and is expressed in the unified atomic mass units (u). Examples of molecular weight computations: CO2, SO2. Tetrahydrofuran borane.