percent by weight

Percent by weight is the label on the concentrated reagent shelf. Hydrochloric acid ships at 37% w/w, sulfuric at 98, ammonia solution at 28, hydrogen peroxide at 30, and in each case the number is grams of pure substance per hundred grams of what the bottle holds. Alloy compositions, ores and cement analyses use the same basis, since a solid has no litres to be a percentage of.

Getting from the label to a molarity takes the density printed beside it: 37% HCl at 1.19 g/mL is $0.37 \times 1190 / 36.46 \approx 12$ mol/L, the "concentrated HCl is 12 molar" of every lab manual. The mass-percent and molarity solvers on this site run that chain, and the density is the input people forget, because the percent sign makes the quantity look as if it should convert on its own. It cannot; a mass fraction knows nothing about volume.

The food industry runs this unit under an assumed name. Degrees Brix, the scale on every refractometer in a winery, sugar mill or juice plant, is defined as percent sucrose by weight: 20 °Bx is 20 g of sugar in 100 g of liquid. A grape must at 24 °Bx fermenting toward roughly 14% alcohol by volume is a conversion between two differently based percentages, mass into volume, which is why the factor is an untidy 0.59 and not anything a percent sign would suggest.