Concentration describes the amount of a substance (the solute) dissolved in a specific amount of another substance (the solvent). The resulting homogeneous mixture is called a solution. Expressing concentration numerically is fundamental to chemistry, medicine, and manufacturing, allowing for precise control over chemical reactions and product formulations. Scientists use several distinct units because no single unit is suitable for every application. The unit chosen depends on whether the measurement needs to account for the number of particles, mass, volume, or dilution level.
Concentration Units Based on Moles
Units based on the mole concept quantify the number of particles present in a solution. This is particularly useful for chemical reactions and stoichiometry, as the mole represents a specific, large number of atoms or molecules.
Molarity (\(M\)) is defined as the number of moles of solute dissolved per liter of the total solution volume. The formula is \(\text{M} = \text{moles of solute} / \text{liters of solution}\). Molarity is the most commonly used unit in laboratory settings because it simplifies measuring the required amount of substance using volumetric glassware.
Molarity has a limitation: since it relies on the total volume of the solution, its value changes slightly with temperature. As the solution heats up, the liquid expands, increasing the total volume and lowering the molarity value. While often negligible at room temperature, this temperature dependence is a concern in experiments spanning a wide temperature range.
Molality (\(m\)) addresses the temperature dependence of molarity by using mass instead of volume. It is defined as the number of moles of solute divided by the mass of the solvent in kilograms, with the formula \(\text{m} = \text{moles of solute} / \text{kilograms of solvent}\). Since mass is constant regardless of temperature, molality provides a stable measure of concentration.
Molality is the preferred unit for studying colligative properties, which are physical properties of solutions depending solely on the number of solute particles present. These properties include the depression of the freezing point and the elevation of the boiling point. For example, calculations for determining the amount of salt needed to melt ice on roads rely on molality for accuracy.
Concentration Units Based on Percent Ratios
Percent concentrations are widely used outside of research laboratories because they are simple to prepare and easy to understand for commercial and medical applications. These units express the amount of solute as a fraction of the total solution, multiplied by 100. The three common types specify whether the components are measured by mass or by volume.
Mass percent (\(\text{w}/\text{w}\%\)) expresses the mass of the solute divided by the total mass of the solution, multiplied by 100. This unit is popular in industrial and manufacturing contexts, such as in the formulation of cosmetics or alloys, where components are typically weighed. For example, a concentrated acid labeled \(37\%\text{ w}/\text{w}\) indicates that 37 grams of acid are present in every 100 grams of the solution.
Volume percent (\(\text{v}/\text{v}\%\)) is used when both the solute and the solvent are liquids. It expresses the volume of the solute divided by the total volume of the solution, multiplied by 100. This unit is commonly encountered on labels for alcoholic beverages, where the alcohol content is specified as alcohol by volume (ABV). A wine labeled \(12\%\text{ v}/\text{v}\) contains 12 milliliters of ethanol per 100 milliliters of wine.
The third type is mass/volume percent (\(\text{w}/\text{v}\%\)), which combines a mass measurement for the solute with a volume measurement for the total solution. It is calculated by dividing the mass of the solute in grams by the volume of the solution in milliliters and multiplying the result by 100. The \(\text{w}/\text{v}\%\) is the standard in many biological and medical environments.
For example, physiological saline solution, used for intravenous fluids, is standardized at \(0.9\%\text{ w}/\text{v}\) sodium chloride. This concentration indicates that 0.9 grams of salt are dissolved to make a final volume of 100 milliliters of solution. This unit is straightforward to prepare in a medical setting, requiring only the weighing of the solid solute and adding enough solvent to reach the final target volume.
Concentration Units for Trace Quantities
When a solute is present in an extremely small amount, such as in environmental monitoring or contamination testing, units based on large ratios are necessary. These “parts-per” notations measure trace quantities where a percentage would require excessive decimal places. They are used for concentrations too dilute for practical molarity or mass percent measurements.
Parts per million (\(\text{ppm}\)) represents one part of the solute per one million parts of the total solution. In aqueous solutions, \(1\text{ ppm}\) is often approximated as one milligram of solute per liter of water. This unit is frequently used to measure air quality, such as atmospheric carbon dioxide, or to check chlorine levels in a swimming pool.
For even more minute concentrations, parts per billion (\(\text{ppb}\)) is employed, representing one part of solute per one billion parts of the solution. \(\text{Ppb}\) is a thousand times smaller than \(\text{ppm}\), offering the necessary precision for highly sensitive measurements. Regulatory agencies, such as those overseeing drinking water safety, use \(\text{ppb}\) to set limits for harmful contaminants like heavy metals.
Selecting the Appropriate Unit
The choice of concentration unit is determined by the context of its use and the physical properties of the measurement. In a chemistry laboratory focused on reaction stoichiometry, molarity is the default choice. This is because chemical equations are balanced based on the number of reacting particles, which the mole directly quantifies. Since laboratory work involves precise volume measurements, the moles-per-volume nature of molarity is convenient for preparation.
For applications where temperature fluctuations are expected or where colligative properties are investigated, molality is the rigorous choice. Because molality is defined using the mass of the solvent, it eliminates the error introduced by volume changes that occur with temperature variation. This unit provides a stable, temperature-independent measure of particle concentration.
In commercial and practical settings, the simplicity of percent concentrations makes them preferable. Mass percent is favored for manufacturing processes involving solid mixtures or where weighing components is easier than measuring their volume. Conversely, the \(\text{w}/\text{v}\%\) unit is a practical compromise in medicine, allowing for the quick preparation of solutions by dissolving a measured mass of solid into a final target volume.