concentration

Meaning: Concentration tells how much solute is present in a given quantity of solution. Common forms are molarity (mol solute per L solution), molality (mol solute per kg solvent), mole fraction (moles of one component divided by total moles, unitless), and mass percent. Molarity changes with temperature because the volume changes. Molality does not, because mass does not change. Concentration sets reaction rates, equilibrium quotients, colligative effects, and pH.

Equation: M = n/V, where M is molarity (mol/L), n is moles of solute (mol), and V is volume of solution (L; SI unit m³). For dilution, M₁V₁ = M₂V₂, where the subscripts mark the initial and final solution. Both volumes must use the same unit.

On the MCAT: Expect dilution and titration calculations, and comparisons of Q with K after a concentration change. Le Châtelier questions ask which way a system shifts when a reactant is added. Rate-law passages ask how rate changes when a concentration doubles. Spectroscopy questions use Beer-Lambert, where absorbance is proportional to concentration. Colligative property problems use molality or the van't Hoff factor times molarity.

Common confusion: Concentration is not the amount. Adding solvent lowers concentration but leaves the moles of solute unchanged. Molarity uses liters of solution, whereas molality uses kilograms of solvent. Square brackets mean molar concentration, and pure solids and liquids are left out of equilibrium expressions.