Current convention defines 1 amu as exactly 1/12 the mass of an atom of the isotope 12C. Since most elements occur naturally in well-established fixed relative abundances of different isotopes, the so-called Atomic Mass for a particular element represents a weighted average of the masses of the individual isotopes. For example, naturally occuring carbon is a mixture of 98.90% 12C (12.000000 amu) and 1.10% 13C (13.003355 amu) plus a trace of 14C (14.003241 amu). The Atomic Mass of carbon is found to be 12.011 amu.
A Molecular Mass is the sum of the (averaged) Atomic Masses for all atoms in a molecule.
What is the Molecular Mass of H2SeO3?