Different ways of expressing the concentration of solutions

Mass Percentage

The mass percentage of a component in a given solution is the mass of the component per 100g of the solution. For e.g., if WA is the mass of the component A, WB is the mass of the component B in a solution. Then,

mass percentage of a component

Example: A 10% solution of sodium chloride in water (by mass) means that 10g of sodium chloride are present in 100g of the solution.

Volume percentage

This unit is used in case of a liquid dissolved in another liquid. The volume percentage is defined as the volume of the solute per 100 parts by volume of solution.

For e.g., If VA is the volume of component A present is Vsol volume of the solution.

Then,

volume percentage of solute

For e.g., a 10% solution of ethanol C2H5OH, in water (by volume) means that 10cm3 of ethanol is present in 100cm3 of the solution.

Strength of a solution is defined as the amount of the solute in gms, present in one litre of the solution. It is expressed as gL-1.

Mathematically,

strength of solution

Molarity

Molarity of a solution is defined as the number of moles of solute dissolved per litre of solution.

Mathematically,

molarity of the solution

For e.g., If 'a' is the weight of the solute (in gms) present in VCC volume of the solution.

Then,

molarity in terms of weight

Molarity is expressed by the symbol M. It can also be expressed as,

molarity in terms of strength

Normality

Normality of a solution is defined as the number of gram equivalents (gm.e) of a solute dissolved per litre of the given solution.

Mathematically it is,

normality

For e.g., If a is the weight of the solute (in gms) present in VCC volume of the solution. Then,

normality in terms of weight

Normality is expressed by the symbol N. It can also be expressed as,

normality in terms of strength

Relationship between molarity and normality

The molarity and normality of a solution is related to each other as follows:

Relationship between molarity and normality

Molality

Molality of a solution is defined as the number of moles of solute dissolved in 1000g of a solvent. Mathematically, it is expressed as

molality

Molality is expressed by the symbol m.

Molality does not change with temperature.

Formality

In case of ionic compounds like KCl, CaCO3 etc. Formality is used in place of molarity.

It is the number of gram formula masses of solute dissolved per liter of the solution. It is denoted by the symbol F. Mathematically it is given as,

formality

Mole Fraction

It is the ratio of number of moles of one component (solute or solvent) to the total number of moles of all the components (solute and solvent) present in the solution. It is denoted by the symbol X. Let us suppose that a solution contains two components A and B and suppose that nA moles of A and nBmoles of B are present in the solution then,

mole fraction of solute

mole fraction of solvent

Adding eq (i) and (ii) we get

xA + xB = 1

Parts per million (ppm)

When a solute is present in very small amounts, its concentration is expressed in parts per million. It is defined as the amount of the solute present in one million parts of the solution.

formula for ppm

It may be noted that the concentration units like molarity, mole fraction etc. are preferred as they involve the weight of the solute and solvent, which is independent of temperature. But units like, molarity, Normality etc., involve volume of the solution, hence changes with temperature.