By Kevin Beck, Updated Mar 24, 2022
If you know the volume and molarity of a solution, you can quickly determine the millimoles (mmol) of the solute present. From that value, you can convert to milligrams (mg) and ultimately calculate the concentration in parts per million (PPM).
While molarity is expressed in moles per liter (mol/L), PPM is a mass‑per‑volume metric. One PPM equals one milligram of solute in one liter of solution—essentially mg/L. The term reflects that the denominator is one million times the numerator in standard units.
The molar mass of potassium (K) is 39.098 g/mol. Thus, 1 mmol of potassium weighs 39.098 mg.
Volume = 0.5 L (500 mL). With a concentration of 0.1 M: \[\text{Moles} = 0.5\text{ L} \times 0.1\text{ mol/L} = 0.05\text{ mol}\]
Since 1 mol = 1,000 mmol, the sample contains 50 mmol of potassium.
Mass = 50 mmol × 39.098 mg/mmol = 1,954.9 mg (≈1,955 mg).
PPM = mg/L. With 1,955 mg in 0.5 L: \[\text{PPM} = \frac{1,955\text{ mg}}{0.5\text{ L}} = 3,910\text{ PPM}\]
PPM is especially useful for measuring trace amounts of substances—often toxic or environmentally significant—within a large volume of solvent. For solids such as soil or gases, related units include PPMm (mass per mass) and PPMv (volume per volume).
By following these steps, you can reliably convert millimoles to PPM for any aqueous solution.
