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Before this: Chemical formulas, The mole

Molar mass

The molar mass of a substance is the mass of one mole of it, in grams per mole (g/mol).

For an element, it is the atomic mass from the periodic table: one mole of carbon has a mass of 12.011 g.

For a compound, add up the atomic masses of every atom in the formula.

Worked example: H2O

ElementAtomsAtomic massSubtotal
H21.0082.016
O115.99915.999

Molar mass = 2.016 + 15.999 = 18.015 g/mol

Parentheses

A subscript after parentheses multiplies everything inside. Ca(OH)2 contains 1 Ca, 2 O and 2 H.

40.078 + 2(15.999) + 2(1.008) = 74.09 g/mol

Common mistake

Forgetting to distribute the outer subscript. In Al2(SO4)3 there are 3 × 4 = 12 oxygen atoms, not 4.

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Watch The molar mass of Ca(OH)2
  1. Ca(OH)2 has 1 calcium atom: 40.078 g/mol.
  2. The subscript outside the parentheses makes it 2 oxygen atoms: 31.998 g/mol.
  3. And 2 hydrogen atoms: 2.016 g/mol.
  4. Add them up: 74.09 g/mol.

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Practice

1. What is the molar mass of water, H2O?

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2. What is the molar mass of calcium hydroxide, Ca(OH)2?

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3. What is the molar mass of glucose, C6H12O6?

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4. In Al2(SO4)3, how many oxygen atoms are in one formula unit?

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5. What is the molar mass of ammonium sulfate, (NH4)2SO4?

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