Concentration Definition (Chemistry)

What Concentration Means in Chemistry

In a solution, concentration is the amount of solute per volume of solvent.
In a solution, concentration is the amount of solute per volume of solvent. Glow Images, Inc / Getty Images

Concentration Definition

In chemistry, concentration refers to the amount of a substance per defined space. Another definition is that concentration is the ratio of solute in a solution to either solvent or total solution. Concentration usually is expressed in terms of mass per unit volume. However, the solute concentration may also be expressed in moles or units of volume. Instead of volume, concentration may be per unit mass. While usually applied to chemical solutions, concentration may be calculated for any mixture.

Two related terms are concentrated and dilute. Concentrated refers to chemical solutions that have high concentrations of a large amount of solute in the solution. Dilute solutions contains a small amount of solvent compared with the amount of solvent. If a solution is concentrated to the point where no more solute will dissolve in the solvent, it is said to be saturated.

Unit Examples of Concentration: g/cm3, kg/l, M, m, N, kg/L

How to Calculate Concentration

Concentration is determined mathematically by taking the mass, moles, or volume of solute and dividing it by the mass, moles, or volume of the solution (or less commonly, the solvent). Some examples of concentration units and formulas include:

  • Molarity (M) - moles of solute / liters of solution (not solvent!)
  • Mass Concentration (kg/m3 or g/L) - mass of solute / volume of solution
  • Normality (N) - grams active solute / liters of solution
  • Molality (m) - moles of solute / mass of solvent (not mass of solution!)
  • Mass Percent (%) - mass solute / mass solution x 100% (mass units are the same unit for both solute and solution)
  • Volume Concentration (no unit) - volume of solute / volume of mixture (same units of volume for each)
  • Number Concentration (1/m3) - number of entities (atoms, molecules, etc.) of a component divided by the total volume of the mixture
  • Volume Percent (v/v%) - volume solute / volume solution x 100% (solute and solution volumes are in the same units)
  • Mole Fraction (mol/mol) - moles of solute / total moles of species in the mixture
  • Mole Ratio (mol/mol) - moles of solute / total moles of all other species in the mixture
  • Mass Fraction (kg/kg or parts per) - mass of one fraction (could be multiple solutes) / total mass of the mixture
  • Mass Ratio (kg/kg or parts per) - mass of solute / mass of all other constituents in the mixture

Some units may be converted from one to another, however, it's not always a good idea to convert between units based on the volume of solution to those based on mass of solution (or vice versa) because volume is affected by temperature.

Strict Definition of Concentration

In the strictest sense, not all means of expressing the composition of a solution or mixture are termed "concentration". Some sources only consider mass concentration, molar concentration, number concentration, and volume concentration to be true units of concentration.