# Normality, Molality, Molarity, Mole fraction, Percentage Solution (%), parts per million (ppm) | Chemistry for Biologists

Normality, Molality, Molarity,  Mole fraction, Percentage solution, Parts per million

Define Molarity(M)

Molarity is defined as the number of moles of solute per litre of solution. The symbol for Molarity is ‘M’.

Molarity (M)= Number of moles of solute  Litre of solution

Define Molality(m)

Molality is defined as the number of moles per kilogram of solvent in a solution. The symbol for molality is ‘m’. It is defined in terms of solvent and not solutions.

Molality (m)= n/w where ‘n’ is number of moles of solute of a substance and ‘w’ is the weight of the solvent in kilograms.

Molality (m)=Number of moles of solute / Kg of water

Normality (N)

Defined as the number of equivalents of solute per litre of solution.

Its unit is normal.

Example: A solution of 5.0 normal is denoted as 5.0N

Mole Fraction (X)

Mole fraction of a substance in a solution is the ratio of the number of moles of that substance to the total number of moles in the solution.

The symbol of mole fraction of A is usually XA .. Thus, for a solution containing X mol of A, Y mol of the B and Z mol of C the mol fraction of A is:

XA  = x mol of A

x mol of A + x mol of B + x mol of C