The relative molecular mass, Mr, is the weighted mean mass of a molecule compared with one twelfth of the mass of an atom of carbon-12.It is found by adding together the relative atomic masses of all the atoms in the molecular formula. For example, water H2O has Mr = (2 × 1.0) + 16.0 = 18.0, and glucose C6H12O6 has Mr = 180.0.The term is used for substances made of simple molecules. For giant ionic compounds such as sodium chloride, which have no molecules, the term relative formula mass is used instead.Mr has no units because it is a ratio of masses. It can be measured using a mass spectrometer, where the molecular ion peak with the highest m/z value gives the relative molecular mass.
Relative Molecular Mass
Read the text
Read the text and highlight anything you think is important. When you go on, the text is hidden and you answer from memory.
Specification: OCR Chemistry A (H032), AS-level, 2.1.3 Amount of substance. Topic: Amount of Substance.
Key terms in this lesson
- molecular ion
- The positive ion, M+, formed in a mass spectrometer when a molecule loses one electron; its mass gives the relative molecular mass.
- mass-to-charge ratio
- The ratio of the mass of an ion to its charge, given the symbol m/z, used to identify ions in a mass spectrum.
More in Amount of Substance
- Molar Mass
- Empirical Formula
- Empirical Formula Calculation
- Relative Formula Mass
- Molecular Formula
- Hydrated Salts
- Solutions and Concentrations
- Standard Solutions
All 16 lessons in Amount of Substance · All OCR AS-level Chemistry topics