APChemistryCollege BoardRevision NotesUnit 1: Atomic Structure & Properties1.1 Moles & Molar MassMolar Mass

Molar Mass (College Board AP Chemistry)

Revision Note

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Molar Mass

The mass of a substance (m), the number of moles it contains, and its molar mass are related by

$n = \frac{m}{M}$

Worked example

How many molecules of C₆H₁₄ does a 42.0 g sample of C₆H₁₄ contain?

Answer:

Molar mass of C₆H₁₄
- M = (6 × 12.01 g/mol) + (14 × 1.008 g/mol)
- M = 72.06 g/mol + 14.112 g/mol
- M = 86.172 g/mol
Number of moles of C₆H₁₄
- $n = \frac{m}{M}$
- $n = \frac{42.0 g}{86.172 g / mol}$
- $n = 0.4874 mol$
Number of molecules of C₆H₁₄
- Number of molecules = N_A × n
- Number of molecules = 6.022 × 10²³mol^-1 × 0.4874 mol
- Number of molecules = 2.94 × 10²³

Alternate Answer

Molar mass of C₆H₁₄
- M = (6 × 12.01 g/mol) + (14 × 1.008 g/mol)
- M = 72.06 g/mol + 14.112 g/mol
- M = 86.172 g/mol
Solve for number of molecules of C₆H₁₄ using the factor-label method
- $42.0 g \times \frac{1 mol}{86.172 g} \times \frac{6.022 \times 10^{23}}{1 mol} = 2.94 \times 10^{23}$

Exam Tip

The factor-label method uses conversions between units that are equivalent to convert from one unit to another. The original number is multiplied by a fraction made up of the equivalent units so that the units of the original number cancel with one of the units in the conversion. If this leads to the units for the answer then the problem is complete. If not, another conversion can be added.

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Author: Fallon

Fallon obtained a double major in chemistry and secondary education, and after graduating she taught Chemistry and Organic Chemistry for 7 years. Fallon’s passion for creating engaging classroom materials led her to pursue a career in content development. For over 3 years, Fallon has created videos, review materials, and practice questions for AP Chemistry, IGCSE, and other international exam boards.