Porcentagem em massa (m/m)

Mass percent (% w/w): solute concentration by mass

Mass percent (%m/m or % w/w) takes the mass of solute, divides it by the total mass of the solution, and multiplies by 100: %m/m = (msolute / msolution) · 100%. Reading the number off a label is easy enough. Vinegar 5% holds 5 g of acetic acid in every 100 g of solution. Antiseptic alcohol 70% (ANVISA RDC 1.156/2024) carries 70 g of ethanol per 100 g, and saline 0.9% works out to 0.9 g of NaCl per 100 g. Where people trip up is confusing this with %v/v (volume/volume): the two values agree only when the densities match. Brazilian cosmetic and food labels are required to declare the % w/w.

Applications

You'll see it on industrial labels (food, cosmetics, household products) and all over pharmaceutical formulation for creams, ointments and syrups. Plenty of practical chemistry leans on it too, from cleaning products to dye baths, plus quality control on brines and metal alloys. Since it treats solids, liquids and pastes the same way, it's usually the practical choice whenever measuring volume would be a hassle.

FAQ

% w/w vs % v/v? Mass stays put when temperature changes, but volume does not. With liquid mixtures such as alcohol, a label could report either one, so check which number you're actually looking at.

How to convert to molarity? Start with the solution's density, which lets you turn mass into volume. After that, M = (% · ρ · 10) / MW (with ρ in g/mL).

ppm and ppb? Same concept on a much smaller scale, for trace amounts: ppm = mg/kg and ppb = µg/kg.