Molecular Mass
The sum of the relative atomic masses of all atoms shown in a molecular formula.
Formula / equation
M_r = Σ A_r (all atoms)
Unit
Dimensionless
In depth
Molecular mass is a pure number on the carbon-12 scale; multiplying it by g/mol gives the molar mass. Mass spectrometry measures it directly by deflecting ionised molecules in a magnetic field, and the value is often the first clue to an unknown compound's identity.
Examples in the real world
Ethanol C₂H₅OH has M_r = 2(12.01) + 6(1.008) + 16.00 = 46.07; ammonia NH₃ has M_r = 17.03.