Molar mass — Science definition
Defined in 2 courses:
- The mass of one mole of a substance in grams, numerically equal to the sum of the atomic masses in its formula (g/mol). (Physical Science: Chemistry)
- The mass of one mole of a substance in grams, found by adding the atomic masses from the periodic table. (Physical Science: Chemistry)
Where it is taught
- The Mole & Molar Mass — Reactions, Moles & Composition, Physical Science: Chemistry, Grade 11
- Concentration & Molarity — Solutions & Dissolved Particles, Physical Science: Chemistry, Grade 11
In use
The molar mass of a substance — the mass of one mole of it — is numerically equal to the sum of the atomic masses in its formula, just with the unit grams per mole.
The Mole & Molar Mass
To use molarity you often have to start from a mass on a balance, not a number of moles, so you need one extra step: convert grams to moles by dividing by the substance's molar mass (the mass of one mole, read off the periodic table).
Concentration & Molarity
Questions using molar mass
Problem 1
Calculate the molar mass of carbon dioxide, CO₂. (C = 12.01, O = 16.00 g/mol.)
Answer and explanation
44.01 g/mol
Molar mass = 12.01 + 2(16.00) = 12.01 + 32.00 = 44.01 g/mol.
Problem 2
How many moles are in 88.02 g of carbon dioxide? (Molar mass CO₂ = 44.01 g/mol.)
Answer and explanation
2 mol
moles = mass ÷ molar mass = 88.02 g ÷ 44.01 g/mol = 2.00 mol.
Problem 3
What is the mass of 3.00 mol of water? (Molar mass H₂O = 18.02 g/mol.)
Answer and explanation
54.06 g
mass = moles × molar mass = 3.00 mol × 18.02 g/mol = 54.06 g.
Problem 4
Calculate the molar mass of glucose, C₆H₁₂O₆. (C = 12.01, H = 1.01, O = 16.00 g/mol.)
Answer and explanation
180.18 g/mol
6(12.01) + 12(1.01) + 6(16.00) = 72.06 + 12.12 + 96.00 = 180.18 g/mol.