Mole Concept
Move accurately among mass, moles, particles, atoms, formula units, and gas volume while explaining why each conversion factor is used.
Mole Concept
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The mole is a counting bridge
One mole contains 6.02×10²³ representative particles. The representative particle depends on the substance: atoms for a monatomic element, molecules for a molecular substance, and formula units for an ionic compound.
moles = mass ÷ molar massparticles = moles × 6.02×10²³For school-level problems at STP, one mole of an ideal gas is commonly approximated as 22.4 L.
Molar mass links grams and moles
Add the atomic masses shown in the chemical formula.
Avogadro’s number links moles and particles
Multiply going from moles to particles; divide going back.
Subscripts count particles within particles
One CO₂ molecule contains two oxygen atoms.
Ionic compounds use formula units
NaCl is counted in formula units, not discrete molecules.
Gas volume needs stated conditions
Use 22.4 L per mole only when the problem specifies the school-standard STP model.
Use the mole road map
Grams to moles: divide by molar mass.
Moles to grams: multiply by molar mass.
Moles to particles: multiply by 6.02×10²³.
Particles to moles: divide by 6.02×10²³.
Only after reaching molecules or formula units should you use a formula subscript to count particular atoms or ions.
Why it works
Moles keep extremely large particle counts manageable and allow chemical formulas and equations to describe measurable laboratory amounts.
Five forms you should recognize
Problem: How many moles are in 18 g H₂O? Use H = 1 and O = 16.
molar mass H₂O = 2(1) + 16 = 18 g per molemoles = 18 g÷18 g per mole = 1 molProblem: How many molecules are in 0.50 mol O₂?
0.50 mol×6.02×10²³ = 3.01×10²³ moleculesProblem: Find the mass of 1.204×10²⁴ CO₂ molecules.
moles = (1.204×10²⁴)÷(6.02×10²³) = 2 molmass = 2 mol×44 g per mole = 88 gProblem: How many hydrogen atoms are in 3.01×10²³ CH₄ molecules?
Each molecule has four H atoms.
4(3.01×10²³) = 1.204×10²⁴ H atomsProblem: How many moles occupy 11.2 L at STP using 22.4 L per mole?
moles = 11.2÷22.4 = 0.50 molWhy: The stated STP model supplies the volume-to-mole conversion.
Check before you commit
- Multiplying by molar mass when converting grams to moles
- Calling ionic formula units molecules
- Forgetting formula subscripts when counting atoms
- Using Avogadro’s number as a molar mass
- Using 22.4 L per mole without stated gas conditions
- Rounding too early in a multistep conversion
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Mole Concept FAQ
What is a representative particle?
It is the entity being counted: an atom, molecule, formula unit, ion, or another specified particle.
Why is NaCl not counted in molecules?
Solid NaCl forms an ionic lattice rather than separate NaCl molecules.
How is this different from stoichiometry?
This page builds conversions for one substance. Stoichiometry uses balanced-equation mole ratios to connect different substances.
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