Mass % = (mass of solute / mass of solution) × 100
Mass percent (w/w%) expresses concentration as the mass of solute divided by the total mass of solution, multiplied by 100. It is temperature-independent (unlike molarity, which changes with volume expansion). Examples: 3.5% NaCl = seawater, 70% isopropanol = rubbing alcohol, 96% H₂SO₄ = concentrated sulfuric acid. To convert to molarity: M = (mass% × density × 10) / molar mass. Mass percent is widely used in: pharmaceutical formulations, food labeling (e.g., alcohol by weight), alloy composition (e.g., 18/8 stainless steel = 18% Cr, 8% Ni), and industrial chemistry. Related units include parts per million (ppm = mg/kg) and parts per billion (ppb = μg/kg) for trace concentrations.