Relative molecular mass is calculated based on which comparison?

Relative molecular mass, also known as molecular weight, is defined as the weighted mean mass of a molecule compared to 1/12 of the mass of carbon-12. This comparison is important because the carbon-12 isotope serves as a standard reference point in chemistry for measuring atomic and molecular masses.

By using carbon-12 as a benchmark, scientists can express the masses of other atoms and molecules in a consistent way. The mass of a molecule is determined by taking into account the relative masses of all constituent atoms, weighted by their respective abundance if they occur in different isotopes. This method allows for accurate calculations and comparisons between different molecular structures.

Other options do not accurately describe how relative molecular mass is calculated. For instance, comparing to hydrogen is not standardized in the same way as using carbon-12. The sum of atomic masses is part of the process but does not include the critical aspect of the weighted mean. Furthermore, the concept of weight of molecules in moles pertains to a different calculation and does not reflect the definition of relative molecular mass. Thus, the correct definition relates specifically to the comparison with carbon-12.