Methods of Expressing the Strength of Solution

Strength of solution is the amount of solute in grams dissolved in one litre of solution. It is generally expressed in gram/litre.

Other methods of expressing the strength of solution are

(A) Mass percentage:
Mass % of solute =  x 100

Mass % of solvent =  x100

(B) Volume percentage:
Volume % of solute =  x 100

Volume % of solvent =  x 100

(C) Molatity (m):
 It is no. of moles of solute dissolved into 1 kg of the solvent.

m = 

(D) Molarity (M):
It is no. of moles of solute dissolved in 1 litre of solution

M = 

(E) Normality (N):
It is no. of gram-equivalents of solute dissolved in 1 litre of solution

N = 

(F) Formality:
Ionic solutes do not exist in the form of molecules (It is only covalent molecules that exist in the form of molecules). Therefore they do not have a molecular mass. Instead they have Gram-formula mass. Therefore, molarity for ionic compounds is actually called as formality. It is defined as the no. of gram formula mass of solute dissolved in 1 litre of solution.

F = 
 

Note: When formula weight is equal to molecular weight, formality is equal to molarity.

(G) Mole fraction:
Mole fraction of solute = 

Mole fraction of solvent = 

For a solution, mole fraction of solute + mole fraction of solvent = 1.

(H) Mass fraction:

Mass fraction of solute = 

Mass fraction of solvent = 

(I) Parts per million (ppm): It is defined in two ways
ppm = mole fraction x 10⁶
ppm = mass fraction x 10⁶
 

Note: (1) There is no effect of temperature on molality, mole fraction and mass fraction but a significant effect on molarity, normality and volume fraction. [Since volume is affected by temperature (Charle's law)]
(2) Relationship N = M x n - factor M =  m =  M =	where N =  Normality M = Molarity m = Molality d = Density of solution M2 = Molecular mass of solute x2 = Mole fraction of solute x1 = Mole fraction of solvent M1 = Molecular mass of solvent