Molarity
The International Union of Pure and Applied Chemistry (IUPAC) defines
concentration as:
the amount of substance per unit volume.
And IUPAC also defines amount of substance in moles.
The SI symbol for the mole is “mol”.
For most volumetric analysis purposes the molar mass is the same as the old atomic weight and molecular weight based on hydrogen.The mole can be thought of as:
The mole represents a number of atoms or molecules equal to Avogadro’s
number. Avogadro’s number (6.022x1023 ) is the number of atoms in exactly 12 g of carbon -12. Thus,
6.022 1023 atoms or molecules = 1 mol
Mole can also be related to molar mass. The molar mass of a substance is the mass in grams of one mole of that substance.
For an element, the molar mass is the mass in grams numerically equal to the relative atomic mass or atomic weight.For a compound, the molar mass is the sum of the related atomic masses of all the atoms that have combined in one molecule or formula unit in grams.
Molar mass has the unit of grams per mole or g mol–1. For example, the molar mass of oxygen is 32.00 g/mol. Molar mass therefore provides the conversion factors that relate the mass in grams and the number of moles of a
substance.
No. of moles (n) = mass/molar mass
or n = m/Mr ... (4.1)
Or mass = (no. of moles) (molar mass)
m = n Mr
This mole concept is also useful in expressing concentration of solution. Theusual unit of volume is the dm3 (litre). Therefore, a one molar solution is defined, as one that contains one mole of solute in each dm3 of a solution.
Molarity = No.of moles of solute/Volume of solution in dm3
No. of moles of solute = molarity × volume of solution
n = M . V
Where M = molarity of the solution
V = volume of the solution in dm3
n = no. of moles of solute.
n = m/Mr
Therefore, m/Mr = M x V
m = Mr × M× V
Where m = gram mass of solute
Mr = molar mass
Normality
This is an old-fashioned method of expressing concentration but you may still come across it in older books and worksheets. It is based on the equivalent of a substance. The equivalent of a substance is : the mass of that substance that contains 1 g of replaceable hydrogen or that will displace 1 g of replaceable hydrogen from an acid.
A normal solution, symbol N, contains one equivalent of solute in 1 dm3 of solution.
A 2N solution contains two equivalents of solute per dm3 of solution,etc..
Molality
The molality of a solution is the number of moles of the solute per kilogram of the solvent contained in a solution. It is given by the following expression:
Molality =( m1×1000)/(m2×Mr)
where,
m1 = mass of solute in kg
m2 = mass of the solvent in kg
Mr = molar mass of the solute in kg/mole
The molality scale is useful for experiments in which physical measurements e.g., freezing point, boiling point, vapour pressure, etc., are made over a wide range of temperatures
Parts Per Million (ppm)
This unit is particularly useful for expressing very small concentrations. We find this unit by using:
ppm =. mass of solute / ×1000000
( mass of solvent+mass of solute)
The masses of solute and solvent should be expressed in the kg unit. The concentrations of air and water pollutants are often given in parts per million.Various ways of expressing concentrations are given here just to make you aware of these. Though in modem text by and large, the concept of molarity is being used, you would come across other expressions also.