When to use molarity, molality, and ppm
Molarity (moles per liter of solution) is the most common unit in the lab because it relates directly to volumes that chemists measure with pipettes and flasks. Molality (moles per kilogram of solvent) stays constant with temperature, making it ideal for calculations involving colligative properties or reactions that occur over a wide temperature range. Parts per million is used for trace quantities in environmental samples, water analysis, and pharmaceutical applications where the concentration is extremely low.
The choice depends on the task. If you are preparing a stock solution and need to know how much solute to weigh out, molarity is most practical. If you are studying freezing-point depression, molality avoids temperature errors. If you are testing for contamination in drinking water, ppm is the standard.
Converting between concentration units
Conversion requires knowing the density of the solution and the molar mass of the solute. Molarity to molality involves the solution density; a 1 M aqueous solution is usually slightly less than 1 m because the solute takes up volume. Moving between molarity and ppm requires accounting for the density of the sample. These conversions are straightforward algebra but easy to misapply if the appropriate density is not known.
Practical limits of each scale
Molarity breaks down at very high solute concentrations where volume is no longer additive. Molality works well across all ranges but requires knowledge of solvent mass rather than solution volume. Parts per million becomes ambiguous at high concentrations because it can mean ppm by mass or ppm by volume, and the relationship between them depends on density. For most general chemistry, molarity suffices; for advanced work, having all three tools available ensures precision.