What Is Molar Mass?
Molar mass is the mass of one mole (6.022×10²³ particles) of a substance, expressed in grams per mole. It's the essential bridge between the mass you weigh on a balance and the number of moles used in chemical calculations.
Molar mass (M) is the mass in grams of one mole of a substance: M = m/n, found by summing the atomic masses of all atoms in a formula (in g/mol).
- •Read the atomic mass under the element symbol
- •Example: Na = 22.99 g/mol
- •Units are already g/mol
- •Sum atomic mass × subscript for each element
- •Example: H2O = 2(1.01)+16.00 = 18.02 g/mol
- •Add all elements together
Try it: interactive calculator
Step-by-step worked examples
Find the molar mass of water, H2O. (H=1.01, O=16.00 g/mol)
M = 2×M(H) + 1×M(O) M = 2×1.01 + 16.00 M = 2.02 + 16.00 = 18.02 g/mol
Find the molar mass of carbon dioxide, CO2. (C=12.01, O=16.00)
M = 1×M(C) + 2×M(O) M = 12.01 + 2×16.00 M = 12.01 + 32.00 = 44.01 g/mol
A sample of NaCl has a mass of 58.5 g and contains 1.00 mol. Find its molar mass and confirm.
M = m/n = 58.5 g / 1.00 mol = 58.5 g/mol Check by formula: Na=22.99 + Cl=35.45 = 58.44 g/mol Values agree (rounding) ✓
Flashcards
Quick quiz
Q1.Molar mass is measured in:
Q2.Molar mass of CO2 (C=12.01, O=16.00)?
Q3.58.5 g of a substance is 1.00 mol. Its molar mass is:
Q4.To find a compound's molar mass you:
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Common mistakes
Forgetting to multiply by the subscript for each element. — Correct: Multiply each element's atomic mass by how many atoms of it are in the formula.
Confusing molar mass with atomic mass. — Correct: Atomic mass is per atom (amu); molar mass is per mole of substance (g/mol) — numerically similar for elements only.
Mixing up units — using kg or mg instead of g. — Correct: Molar mass calculations use grams (g) and moles (mol); convert other units first.
Ignoring parentheses/subscripts in formulas like Ca(OH)2. — Correct: Multiply everything inside parentheses by the outer subscript: 2 O and 2 H from (OH)2.
FAQ
What is molar mass?
The mass of one mole of a substance, expressed in grams per mole (g/mol).
What is the molar mass formula?
M = m/n — mass in grams divided by amount in moles; for compounds, sum each element's atomic mass × its subscript.
Can you give molar mass examples?
Water (H2O) = 18.02 g/mol, carbon dioxide (CO2) = 44.01 g/mol, sodium chloride (NaCl) = 58.44 g/mol.
How do you calculate molar mass from a formula?
Multiply each element's atomic mass by its subscript, then add all the results together.