Unit  of concentration of solution

Unit  of concentration of solution:

Molarity (M): Molarity of a solution is the number of moles of solute dissolved per litre of solution. Thus,

Molarity (M) =  Number of moles of solute/Volume of solution in litres

If W_A gram of solute is present in V ml of a given solution, then-

Molarity (M) = (W_{A/Molar mass} x1000/V)

Molality (m): Molality is the number of moles of the solute dissolved per l000g (or 1 kg) of the solvent. It is denoted by m. Therefore,

Molality (m) =Number of moles of solute/Mass of solvent in 1 kg or l000g

If W_A gram of solute is present in W_B gram of solvent, then

Molality (m) = (W_{A/Molar mass} x1000/W_B)

Mole Fraction (X):

Mole fraction is the ratio of number of moles of solute or solvent to the total number of moles present in the solution. It is denoted by X. Let us consider n_A and n_B be the number of moles of A and B present in solution. Then,

Mole fraction of solute A, X_A =   n_{A /} (n_A + n_B)                                            ————–   (1)

Mole fraction of solute B, X_B = n_B / (n_A + n_B)                                              ————–   (2)

Adding expression (1) & (2), we have

X_A + X_B =  n_{A    / (}n_A + n_B )  +  n_B  /  (n_A + n_B)  =1

Therefore, sum of mole fractions of all the components in solution is always equal to one.

Normality (N): Normality of a solution is the number of gram equivalent of solute dissolved per litre of the given solution. Therefore,

Normality (N) =Number of gram equivalent of solute/Volume of solution in litres

If W_A gram of solute is present in V ml of a given solution, then

                              Molality (m) =  W_A /Eq. mass  x 1000/V

Percentage (%): The mass percentage of a component in a given solution is the mass of the component per 100g of the Solution. The % may be express in the following ways:

• (w/w) %
• (w/v) %
• (v/w) %
• (v/v) %

Raoult’s Law for solution containing non-volatile solute:

According to Raoult’s law, at a given temperature, the vapour pressure of a solution containing non-volatile solute is directly proportional to the mole fraction of the solvent. If P_A is the vapour pressure of a solution containing non-volatile solute and X_B is its mole fraction. Then,

P_A α X_B

Or, P_A = K X_B                    Where, K = Proportionality constant.

For pure solvent, X_B = 1, then K becomes equal to vapour pressure of pure solvent which is denoted by P°_A Therefore,

P_A = P_A° X_B                                        (P_A = P_A°)