25 g of Salt in 100 g of Water Is a 20 % Solution

w/w, w/v and v/v are three different things

A percentage on a bottle can mean grams per hundred grams, grams per hundred millilitres, or millilitres per hundred millilitres, and the three only coincide when the density of the solution is exactly one. Five grams in a hundred millilitres of a solution whose density is 1.1 is 5 g in 110 g, or 4.55 % by mass — nine per cent below the number on the label. Pharmacy and much of clinical practice use grams per hundred millilitres, food labelling generally uses mass, and spirits use volume. Read which of the three is meant before comparing two products or scaling a preparation.

Masses add; volumes do not

This is the practical advantage of working in mass: a hundred grams of water plus twenty-five grams of salt is a hundred and twenty-five grams of solution, exactly, with no measurement needed. Volumes have no such guarantee — mixing equal volumes of ethanol and water gives measurably less than their sum, because the molecules pack together more closely than either does alone. Any preparation defined by adding volumes therefore has to be made up to a final mark rather than computed, while one defined by mass can be weighed out in any order and will come out right.

Frequently asked questions

Does a saturated solution have a fixed mass percent?

For a given substance, solvent and temperature, yes — saturation is a definite point, and it is where the solubility tables are read. Change the temperature and it moves, usually upward for solids in water, which is why a solution saturated while warm deposits crystals as it cools. It is not a universal number: the saturated mass percent of table salt in water and of sugar in water are far apart, and neither tells you anything about a third substance. Look the value up for the exact pair rather than assuming a general figure.

How do I scale a mass percent recipe up?

Decide the final mass you want and take the percentage of it. For 400 g of a 20 % solution you need 80 g of solute and 320 g of solvent, because the two must add to the 400. The mistake is to keep the solvent at its original figure and add more solute, which changes the concentration and gives a total nobody chose. Working from the final mass backwards makes the arithmetic trivial at any scale and removes the need to think about the denominator at all.