Chemistry · Atomic Structure and Properties · 14 min read · Updated 2026-05-11
Moles and Molar Mass — AP Chemistry
AP Chemistry · Atomic Structure and Properties · 14 min read
1. The Mole and Avogadro’s Number★☆☆☆☆⏱ 3 min
The mole is the SI base unit for amount of substance, connecting the microscopic world of atoms, ions, and molecules to the macroscopic masses we measure in the lab. It is abbreviated mol, and should be referred to as "amount of substance" to distinguish it from mass. Avogadro’s number ($N_A$) is the fixed number of elementary entities (atoms, molecules, formula units) per mole.
n = \frac{N}{N_A} \quad \text{or} \quad N = n \times N_A
Exam tip: Always specify the type of particle you are counting in FRQ answers. AP graders will deduct points for vague answers that do not name the particle.
2. Molar Mass of Elements and Compounds★★☆☆☆⏱ 3 min
Molar mass ($M$) is defined as the mass per mole of a substance, with standard units of grams per mole (g/mol). For any element, molar mass is numerically equal to the average atomic mass listed on the periodic table. For compounds, molar mass equals the sum of the molar masses of all atoms in the compound’s chemical formula, including water of hydration for hydrated ionic compounds.
M_{A_xB_y} = xM_A + yM_B
Exam tip: Always confirm you count all atoms in the water of hydration for hydrates. A common mistake is miscounting hydrogen atoms in water molecules.
3. Mass-Mole-Particle Conversions★★☆☆☆⏱ 4 min
The core utility of moles is that they act as an intermediate to convert between three common chemical quantities: measured sample mass, amount of substance in moles, and number of particles. You can never convert directly from mass to number of particles without passing through moles. The two key relationships for conversions are:
To go from mass to number of particles: follow the chain $\text{mass} \rightarrow \text{moles} \rightarrow \text{particles}$
To go from number of particles to mass: follow the chain $\text{particles} \rightarrow \text{moles} \rightarrow \text{mass}$
Exam tip: Always write units for every step of your conversion. If your units do not cancel correctly to give your desired final unit, you have flipped a ratio and can correct it early.
4. Percent Composition by Mass★★☆☆☆⏱ 3 min
Percent composition by mass is the percentage of a compound’s total mass that comes from each individual element. This is an intensive property (independent of sample size) used to identify unknown compounds and as the first step in calculating empirical formulas.
\% \text{X} = \frac{\text{Total mass of X in 1 mole of compound}}{M_{\text{compound}}} \times 100\%
The sum of all percent compositions for a compound will always add up to approximately 100% (small differences from rounding are acceptable), making this a quick check for calculation errors.
Exam tip: Always check that your percent compositions add to ~100% before moving on to downstream calculations like empirical formula determination. This catches small arithmetic errors early.
5. AP Style Concept Check★★☆☆☆⏱ 3 min
Common Pitfalls
Why: Students separate water from the anhydrous compound and accidentally count only one hydrogen per water molecule.
Why: Students confuse Avogadro’s number’s relationship to moles with a relationship to mass.
Why: Students memorize that pure nitrogen is diatomic and incorrectly carry that molar mass into compound calculations.
Why: Students round early to "simplify" calculations.
Why: Students forget that subscripts count atoms per molecule, so they also count moles of atoms per mole of compound.