Mass Percent of a Solution

Mass percent is the concentration measure that needs nothing but a balance: the solute's mass divided by the mass of the whole solution — solute plus solvent — times 100. Dissolve 25 g of salt in 225 g of water and the solution is 25/250 × 100 = 10% salt by mass. The classic mistake is dividing by the solvent mass alone, which would wrongly give 11.1%.

Because it is temperature-proof and instrument-free, mass percent dominates industrial and household labels: household vinegar is about 5% acetic acid, physiological saline 0.9% NaCl, seawater roughly 3.5% dissolved salts, and concentrated sulfuric acid ships at 98%. Converting to molarity requires the solution's density, which is why bottle labels for concentrated acids list both figures side by side.