biophysic course

The Concentrations

The concentration of a solution measures the amount of solute that has been dissolved in a given amount of solvent or solution. There are several ways to express the amount of solute present in a solution, including:

Molar concentration or molarity C

The molar concentration expresses the number of moles of the solute per the volume of the solution. This concentration is measured in mol/m3, in mol/L, or subunits such as mmol/L.

ponderal concentration Cp

The ponderal concentration expresses the mass of the solute per the volume of the solution. it is expressed in g/m3, in g/L, or in subunits such as mg/L.

relation between molar concentration and ponderal concentration

Mass concentration can be calculated using molar concentration, by replacing m=n×M in the mass concentration relation. So, the relation becomes as follows: Cp=C×M

Molal concentration or molality Cml

The molal concentration is defined as the ratio of the number of moles of the solute to the mass of the solvent contained in the sample. it is expressed in mol/kg.

Molar fraction

The molar fraction expression is used to determine the mole ratio of every constituent of the solution. It is defined as the ratio between the number of moles of the constituent ni (solute) on the total number of moles present in the solution ∑ni (solvent + solute).

Mass or weight fraction

The mass fraction expression is used to determine the mass ratio of every constituent of the solution. It is defined as the ratio between the mass of the constituent m_i (solute) on the total mass ∑mi (solvent + solute).

Note :

  • The sum mole fraction equals 1

  • The sum of the weight fraction equals 1

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