The formula
p = (mₛ / mₜ) × 100
| Symbol | Quantity | Unit |
|---|---|---|
| p | Concentration | % |
| mₛ | Mass of solute | g |
| mₜ | Total mass of the solution | g |
What it means
It is the most direct way of stating a concentration and the one that appears on labels: a "3 %" hydrogen peroxide carries three grams of peroxide per hundred grams of solution. The word to underline is **solution**, not solvent: those hundred grams include the solute. Confusing the two is the most frequent error and makes the solution come out stronger than intended.
How to work it out by hand
- Weigh the solute
- Weigh the complete solution, solute included
- Divide the solute by the total
- Multiply by a hundred
What is worth knowing
There are three percentages written the same way that do not mean the same thing, and labels rarely say which they use. Mass by mass (w/w) is the one in this calculator. Volume by volume (v/v) is the one on alcoholic drinks: a 13-degree wine carries 13 ml of ethanol per 100 ml. And mass by volume (w/v), common in clinical labs, is grams per 100 ml. For dilute aqueous solutions all three come out similar because water weighs almost exactly one gram per millilitre, but as soon as the density moves away from one they stop being interchangeable.