The molarity of a solution is the number of moles of solute per liter of solution. The molar mass of any particle (atom, molecule, formula, or ion) is the sum of the average atomic masses of all atoms forming that particle. This is also known as molecular weight.
The molar mass of an atom is its average atomic mass expressed in grams. The atomic mass of all atoms can be found in the periodic table.
For dilute solutions, one part per million equals one mg/L. In most cases, the two are considered equal.
To convert from molarity to ppm, first determine the molar mass of a substance. For instance, chloride has a molar mass of 35.5. A solution with 1 M concentration would have 35.5 g of chloride per 1 L of solution.
| Molarity moles/Liter (M) | grams/L (g/L) | Parts Per Million (ppm) | milligrams/L (mg/L) |
| 1 M | 35.5 | 35,500 | 35,500 |
| 10-1 M (0.1 M) | 3.55 | 3,550 | 3,550 |
| 10-2 M (0.2 M) | 0.355 | 355.0 | 355.0 |
| 10-3 M (0.3 M) | 0.0355 | 35.5 | 35.5 |
| 10-4 M (0.4 M) | 0.00355 | 3.550 | 3.550 |