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1.1Atomic Structure and Properties

Moles and Molar Mass — AP CHEMISTRY Revision Notes & Topical Questions

5 min read 9 parts Lesson 7 of 16 1 quiz

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Lesson summary

1.1 Moles and Molar Mass — Notes — AP CHEMISTRY notes: Calculate quantities of a substance or its number of particles using the mole concept and dimensional analysis.Particles cannot be counted directly in the laboratory.A connection is needed between the masses of reacting subst…

  • Avogadro's number
  • Atoms of one element
  • Molar mass (M)
  • What the symbols mean
  • Atomic mass unit
  • Formula unit

Part 1 of 7Why the mole is needed

Why the mole is needed
Particles cannot be counted directly in the laboratory.
A connection is needed between the masses of reacting substances and the number of particles undergoing change.
The mole provides that link.

Part 2 of 7Avogadro's number

One mole contains exactly 6.022 × 1023 particles, which may be atoms, molecules, ions or formula units.
Avogadro's number
  • NA = 6.022 × 1023 mol−1, so N = n × NA.
For ionic compounds the particle counted is a formula unit, for example CaCl2 contains 1 Ca2+ and 2 Cl−.
Atoms of one element
Atoms of one element moles of compound × subscript × NA.

Part 3 of 7Atomic mass unit and molar mass

The atomic mass unit (amu) is 1/12 of the mass of one carbon-12 atom.
The mass of one particle in amu is numerically equal to the mass of one mole in grams.
Molar mass (M)
Molar mass (M) the mass of one mole of a substance in g/mol.
  • It is the sum of the average atomic masses of all atoms in the formula.
  • For example M(H2O) = 2(1.008) + 16.00 = 18.02 g/mol.

Part 4 of 7Moles from mass

Equation
  • Equation n = m/M, with m in grams and M in g/mol.
    What the symbols mean: n = amount of substance (mol) · m = mass (g) · M or Mr = molar mass (g mol−1, numerically equal to Mr)
This gives a quantitative connection between the mass of a substance and the number of particles it contains.

Part 5 of 7Dimensional analysis

Dimensional analysis
Write every quantity with units and multiply by conversion factors such as g/mol and particles/mol.
Choose each factor so the unwanted units cancel.
Report answers to the significant figures of the least precise measurement; exact counts and defined constants do not limit significant figures.

Part 6 of 7Converting mass to number of particles

1
Write the formula and calculate the molar mass from the periodic table.
2
Divide the mass in grams by the molar mass to get moles.
3
Multiply moles by the subscript if atoms or ions within the formula are required.
4
Multiply moles by 6.022 × 1023 mol−1.
5
Check that units cancel and round to the correct significant figures.

Part 7 of 7More from the specification

Factor-label method
  • Conversion using fractions of equivalent units that cancel.
Sulfate
  • Salt of sulfuric acid.
Oxygen
  • A gas that relights a glowing splint.
Hydrogen
  • A gas that gives a squeaky pop with a lighted splint.

Key terms

TermMeaning
MoleAmount containing 6.022 × 1023 particles.
Avogadro's numberNumber of particles per mole, 6.022 × 1023.
Molar massMass of one mole of a substance in g/mol.
Atomic mass unitOne twelfth of the mass of a carbon-12 atom.
Formula unitSimplest ratio of ions in an ionic compound.
Dimensional analysisMethod using unit conversion factors to solve problems.

Avoid these mistakes

1
Using amu in place of g/mol for molar mass
2
Counting formula units when atoms are required — multiply by the subscript
3
Forgetting that mass is conserved but particle counts are compared through moles
4
Dropping units during the calculation, so errors in cancelling go unnoticed

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