METHODS OF EXPRESSING THE CONCENTRATION OF A SOLUTION
The concentration of a solution can be expresses in a number of ways. The important methods are:
(i) Mass percentage or per cent by mass :
It is defined as the amount of solute in grams present in 100 grams of the solution.
Mass percentage = Mass of solute/Mass of solution × 100
= Mass of solute/Mass of solute+Mass of solvent × 100
=Mass of solute/Volume of solution×Density of solution × 100
The ratio Mass of solute/Mass of solvent is termed as mass fraction.
Mass percentage of solute = Mass fraction × 100
10% solution of sugar means that 10 grams of sugar is present in 100 grams of the solution, i.e., 10 grams of sugar has been dissolved in 90 grams of water.
(i) Percent by volume:
It is defined as the volume of solute in mL present in 10 mL solution.
Per cent of solute by volume =Volume of solute/Volume of solution × 100
(ii) Per cent mass by volume:
It is defined as the mass of solute present in 100 mL of solution.
Per cent of solute mass by volume =Mass of solute/Volume of solution × 100
(iii) Strength or concentration (Grams per litre):
It is defined as the amount of the solute in grams present in one litre of the solution.
Concentration of solution = Mass of solute in grams/Volume of the solution in litres
= Mass of solute in grams/Volume of the solution in mL × 100
Concentration in grams per litre is also termed as strength of the solution. let w g of the present in V litre of solution, then
Strength or concentration of the solution =w/V gL–1
Note: V is not the volume of the solvent. V is actually the final volume after dissolving a definite quantity of solute in the solvent.
(iv) Parts per million (ppm):
When the solute is present in trace quantities, it is convenient to express the concentration in parts per million (ppm). It is defined as the quantity of the solute in grams present in 106 grams of the solution.
ppm = Mass of solute/Mass of solute × 106
Atmospheric pollution in cities is also expressed in ppm by volume. It refers to the volume of the pollutant in 106 untis of volume. 10 ppm of SO2 in air means 10 mL of SO2 is present in 106 mL of air.