Mass concentration (chemistry)

Mass concentration, often denoted by ρ (rho), γ (gamma), or sometimes c, is a measure of the concentration of a substance in a mixture, defined as the mass of a constituent divided by the volume of the mixture. Unlike molality or molarity, it does not rely on the molecular weight of the solute.

Mathematically, mass concentration is expressed as:

ρ = m / V

where:

• ρ is the mass concentration
• m is the mass of the solute
• V is the volume of the solution

The common unit for mass concentration is kilograms per liter (kg/L) or grams per milliliter (g/mL). However, other units such as grams per liter (g/L) and kilograms per cubic meter (kg/m³) are also used, depending on the context.

Mass concentration is a useful concept in various fields, including chemistry, physics, and engineering, especially when dealing with solutions and mixtures where the focus is on the mass of the constituents rather than the number of moles. It is frequently used when dealing with polymers, colloids, and other complex mixtures where the molecular weight is difficult to determine or not relevant.

It is important to distinguish mass concentration from density. Density refers to the mass of the entire mixture divided by its volume, while mass concentration refers only to the mass of the solute divided by the volume of the mixture. In dilute solutions, the mass concentration is often approximately equal to the density of the solution if the mass of the solute is a significant contributor to the overall mass.

Furthermore, mass concentration is related to other concentration measures such as molar concentration (molarity). Molarity is the number of moles of solute per liter of solution. These two concentration measurements can be interconverted if the molar mass of the solute is known using the following relationship:

Molarity = Mass concentration / Molar mass