Cambridge IGCSE Chemistry · Syllabus 0620 · Stoichiometry
Molar mass
What is Molar mass?
Molar mass, M, is the mass of one mole of a substance, measured in grams per mole; its numerical value matches the relative molecular or formula mass, but unlike those quantities it carries a unit and is the value used in the relationship amount of substance equals mass divided by molar mass.
This definition is part of the Stoichiometry chapter in Cambridge IGCSE Chemistry.
Common mistakes with Molar mass
- 7. Treating \(M_r\) and molar mass as the same quantity Repair They share a number and nothing else. \(M_r(\mathrm{CO_2}) = 44\); \(M(\mathrm{CO_2}) = 44\ \mathrm{g\,mol^{-1}}\). Only the second one can go into \(n = m/M\). Lesson 3.2A, Supplement block Supplement 3.3.3
- "\(M_r\) can go straight into \(n = m/M\)." Fix Numerically you get away with it; as a statement it is wrong, because \(M_r\) has no unit. Write the molar mass with \(\mathrm{g\,mol^{-1}}\) and the substitution becomes self-checking.
- Using the reactant's molar mass to convert the product back. Fix Stage 4 uses B's molar mass, not A's. Writing \(M(\mathrm{CaO}) = 56\ \mathrm{g\,mol^{-1}}\) on its own line, with the substance named, keeps the two apart.
- Reading \(\mathrm{g\,dm^{-3}}\) as if it were \(\mathrm{mol\,dm^{-3}}\). Fix Check the unit before choosing a route. A \(\mathrm{g\,dm^{-3}}\) value has to pass through the molar mass before it can meet an equation ratio, because ratios only speak in moles.