Atomic Structure and Properties
Moles & Molar Mass — AP CHEMISTRY Revision Notes & Topical Questions
5 min read 9 parts Lesson 4 of 16 Unit quiz
Lesson summary
Moles & Molar Mass — AP CHEMISTRY notes: Use the mole, Avogadro's number and molar mass to relate mass, amount and number of particles.Mole (mol) = the SI unit of amount of substance.Atoms, ions and molecules are too small to count directly in a laboratory.One mole of any substan…
- Atoms of one element
- Molar mass (M)
- Relationship
- Atomic mass unit
- Formula unit
- Factor-label method
Section 1 of 9The mole concept
The mole concept
Mole (mol)
Mole (mol) the SI unit of amount of substance.
- Atoms, ions and molecules are too small to count directly in a laboratory.
- One mole of any substance contains 6.022 × 1023 particles.
Avogadro's number
- NA = 6.022 × 1023 mol−1.
Number of particles
NA × n.
Avogadro's constant and Avogadro's number are used interchangeably in many textbooks.
Section 2 of 9Specifying the particle
Specifying the particle
One mole of iron contains 6.022 × 1023 Fe atoms.
One mole of water contains 6.022 × 1023 H2O molecules.
- Each molecule has two H atoms and one O atom, so one mole holds 1.2044 × 1024 H atoms and 6.022 × 1023 O atoms.
One mole of an ionic compound contains 6.022 × 1023 formula units.
Section 3 of 9Counting atoms and ions in formulas
Subscripts show how many of each atom or ion are in one formula unit.
Aluminium sulfate, Al2(SO4)3, contains 2 Al, 3 S and 12 O atoms per formula unit.
- One mole has 1.2044 × 1024 Al3+ ions and 1.8066 × 1024 SO42− ions.
Atoms of one element
Atoms of one element moles of compound × subscript × NA.
Section 4 of 9Mass of particles and molar mass
Mass of particles and molar mass
The average mass of an atom, molecule or formula unit is given in atomic mass units (amu).
Molar mass (M)
Molar mass (M) the mass of one mole of a substance, in g/mol.
Molar mass has the same numerical value as the atomic or formula mass in amu.
Molar mass is found by adding the atomic masses of all atoms in the formula.
Section 5 of 9The key equation
The key equation
Relationship
- Relationship n = m/M, with n in mol, m in g 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)
Number of particles
Number of particles n × NA.
Section 6 of 9Factor-label (dimensional analysis) method
1
Write the given quantity with its units.
2
Multiply by conversion factors made of equivalent quantities, such as 1 mol/molar mass in g.
3
Arrange each factor so the unwanted unit cancels.
4
If the result has the wanted unit, stop; otherwise add another conversion.
Factor-label (dimensional analysis) method
A typical route is mass to moles (divide by M) to particles (multiply by NA).
Report the answer to the correct number of significant figures, limited by the least precise measured value.
Section 7 of 9More from the specification
Factor-label method
- Conversion using fractions of equivalent units that cancel.
Oxygen
- A gas that relights a glowing splint.
Hydrogen
- A gas that gives a squeaky pop with a lighted splint.
Section 8 of 9Key terms
| Term | Meaning |
|---|---|
| Mole | Amount of substance containing 6.022 × 1023 particles. |
| Avogadro's number | 6.022 × 1023 particles per mole. |
| Molar mass | Mass of one mole of a substance, in g/mol. |
| Atomic mass unit | One twelfth of the mass of a carbon-12 atom. |
| Formula unit | Simplest ratio of ions in an ionic compound. |
| Factor-label method | Conversion using fractions of equivalent units that cancel. |
Section 9 of 9Exam technique
1
Forgetting to multiply by the subscript when counting atoms or ions per formula unit
2
Using amu where g/mol is needed — molar mass carries g/mol
3
Rearranging n = m/M wrongly — check the units cancel
4
Counting atoms when the question asks for molecules, or the reverse
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