Multiple choiceQuestion 1 · Recall
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By the end you can define relative molecular mass, , and relative formula mass, calculate both from Data Booklet values, and choose the right term for a substance.
Once you know values, the mass of anything is a sum. Water, , has two H and one O:
The only genuinely tricky part is the name: molecular mass or formula mass? That depends on whether the substance is made of molecules at all.
Relative molecular mass, Mr
The weighted mean mass of a molecule compared with one twelfth of the mass of one atom of carbon-12. In terms of the unified atomic mass unit: weighted mean mass of a molecule ÷ 1 u.
Molecules of elements count too: .
Relative formula mass
The weighted mean mass of a formula unit of a compound compared with one twelfth of the mass of one atom of carbon-12.
Used for substances with giant structures, which have no molecules: ionic compounds such as NaCl, and giant covalent compounds such as .
Every element in the molecule is a mix of isotopes, so different water molecules really do have slightly different masses. is the average over all of them, which is why we build it from values (already weighted means).