weight percent
/ weight per-SENT /
When a bottle of vinegar says '5% acetic acid', it means that out of every 100 grams of vinegar, 5 grams are acid. That commonsense reading is exactly what weight percent captures.
Weight percent (also mass percent, written w/w) is the mass of a component divided by the total mass of the mixture, multiplied by one hundred. Because both the part and the whole are measured by weighing, the units cancel and you are left with a pure percentage.
It is a sturdy, temperature-independent way to state concentration, ideal for commercial reagents and solids where weighing is easy and exact. Its limit is that it tells you nothing about particles or volume directly, so converting it to molarity needs the density and the molar mass.
A 30% hydrogen peroxide solution contains 30 grams of pure H₂O₂ in every 100 grams of liquid; the remaining 70 grams are water.
Weight percent is grams of part per 100 grams of whole.
Don't confuse weight percent (w/w) with volume percent (v/v) or weight-per-volume (w/v); for ethanol-water and other liquids, the three give noticeably different numbers for the same bottle.