Chemistry · Grades 9–12
Moles, mass and particle counts
Keep molar mass, mass and amount as different quantities.
Learn the idea
Amount
The mole counts entities. Specify atoms, molecules, ions or formula units.
Mass rule
n = m/M, where mass m and molar mass M use compatible units.
Particles
N = nNₐ. Use a stated rounded value of Avogadro’s constant consistently.
Interpretation
Equal amounts in moles contain equal numbers of the specified entities, not necessarily equal masses.
Worked examples
A sample has mass 18 g and stated molar mass 36 g/mol. How many moles does it contain?
n = 18/36 = 0.5 mol.
0.5 mol
What mass is 0.25 mol of a substance with stated molar mass 80 g/mol?
m = nM = 0.25 × 80 = 20 g.
20 g
Check your understanding
Two different molecular substances each have 1 mol. What must they share?
- The number of molecules.
- Their density.
- Their mass.
Using Nₐ = 6.02 × 10²³ mol⁻¹, how many multiples of 10²³ molecules are in 2 mol?
Using H = 1 and O = 16 g/mol, find the molar mass of H₂O.
Answers and marking notes
Answers and marking notes
- The number of molecules.
A mole contains the same number of specified entities.
- 12.04
2 × 6.02 = 12.04; the full count is 12.04 × 10²³.
- 18 g/mol
2 × 1 + 16 = 18 g/mol.
Further reading and review references
These are reference links, not copied exercises, institutional endorsement or proof of independent review.
Independent educational review required. Grade bands are approximate; these are not officially aligned curricula.