Mass
The basic SI unit of mass is the kilogram (kg). In physics, mass is not the same as weight, even though mass is often determined by measuring the object's weight using a spring scale, rather than balance scale comparing it directly with known masses. An object on the Moon would weigh less than it does on Earth because of the lower gravity, but it would still have the same mass. This is because weight is a force, while mass is the property that (along with gravity) determines the strength of this force.
Molecular mass
Both atomic and molecular masses are usually obtained relative to the mass of the isotope C (carbon 12), which by definition is equal to 12. For example, the molecular mass of methane, whose molecular formula is CH, is calculated as follows:
A more proper term would be "relative molecular mass". However the adjective 'relative' is omitted as it is universally assumed that atomic and molecular masses are relative to the mass of C. Relative atomic and molecular mass values are dimensionless but are given the "unit" Dalton (formerly atomic mass unit) to indicate that the number is equal to the mass of one molecule divided by of the mass of one atom of C. The mass of 1 mol of substance is designated as molar mass. By definition, it has the unit gram.