molarity

Meaning: Molarity (M) is a concentration unit: moles of solute per liter of total solution, not per liter of solvent. A 1 M NaCl solution is made by dissolving 1 mol NaCl and adding water until the final volume is 1 L. Because liquid volume changes with temperature, molarity shifts slightly with temperature, while molality (mol solute per kg solvent) does not. For ionic solutes, ion concentrations follow the formula: 1 M CaCl₂ gives 1 M Ca²⁺ and 2 M Cl⁻ if it dissociates completely.

Equation: M = n/V, where M is molarity (mol/L; mol/m³ in strict SI), n is moles of solute (mol), and V is solution volume (L; m³ in strict SI). Dilution: M₁V₁ = M₂V₂, because the moles of solute stay constant.

On the MCAT: Molarity underlies titrations (moles = M × V), equilibrium expressions for Keq and Ksp, pH from [H⁺], and Beer-Lambert absorbance. Questions ask you to predict that adding solvent lowers M and evaporating solvent raises it, or to find a final concentration after mixing by adding moles and dividing by total volume.

Common confusion: Students mix up molarity with molality, or divide by the solvent volume. Molarity uses the whole solution's volume in liters, so convert mL to L first.