M = ppm / (MW × 1000) for dilute aqueous solutions
Parts per million (ppm) = mg/L for dilute aqueous solutions (where density ≈ 1 g/mL). To convert: Molarity = ppm / (MW × 1000), where MW is in g/mol. Example: 100 ppm NaCl = 100/(58.44 × 1000) = 0.00171 M = 1.71 mM. The factor of 1000 converts mg to g. For concentrated solutions or non-aqueous solvents, the density must be included: Molarity = (ppm × density) / (MW × 1000). PPM is commonly used in environmental monitoring (EPA drinking water standards: lead < 0.015 ppm, arsenic < 0.01 ppm), water treatment, trace analysis, and semiconductor purity specifications. Related units: ppb (parts per billion = μg/L), ppt (parts per trillion = ng/L).