Questions - Answers

Very important questions (4 Marks)

1. Define

           a) Isotonic solutions

           b) Ideal solution

           c) Molal elevation constant (Ebullioscopic constant)

           d) Molal depression constant(Cryoscopic constant).

A: 

           a) The solutions having same osmotic pressure under similar conditions.

           b) A solution which obeys Raoult's law at all conditions of temperatures and concentrations.

           c) The elevation in boiling point observed in one molal solution (Ebullioscopic constant).

           d) The depression in freezing point observed in one molal solution (Cryoscopic constant).

2. Define
             a) Osmosis
             b) Osmotic pressure
             c) Depression in freezing point
             d) Elevation of boiling point.
A: 
             a) The process of inflow of solvent from dilute solution to concentrated solution.
             b) The pressure required to be applied on the solution to prevent osmotic flow of solvent into solution.
             c) The difference between freezing point of pure solvent and freezing point of solution. 
                 Δ T_f = T_f^o − T_f.
             d) The difference between boiling point of solution and boiling point of pure solvent. 
                 Δ T_b = T_b − T_bo.

3. Calculate the equivalent weights of
    a) H₂SO₄        b) Na₂CO₃        c) KMnO₄ in acid medium        d) K₂Cr₂O₇ in acid medium.

4. What are colligative properties? Give 2 examples.
A: The properties of a dilute solution that depends on the number of solute particles.
e.g.: Osmotic pressure, Elevation of boiling point.

5. State Raoult's law. Calculate the vapour pressure of a solution containing 10 g of a non volatile solute present in 80 g of ethanol at 298 K. Given the molecular weight of the solute as 120; the vapour pressure of alcohol is 22.45 mm of Hg at 298 K.
A: The relative lowering of vapour pressure of a dilute solution is equal to mole fraction of non volatile solute.

                     
For dilute solution 

                                

6. a) What is the mole fraction of methanol, if a solution contains 90 g of H₂O and 6.4 g of methanol?
    b) Find the volume of water to be added to 250 ml of 0.05 N Na₂CO₃ solution to make it 0.01 N solution.
A:

        
     ∴  Volume of water = V₂ - V₁ = 1250 - 250 = 1000 ml

7. What is an ideal solution?
A: A solution which obeys Raoult’s law at all the conditions of concentration and temperature is called an ideal solution.
* Enthalpy change of mixing (∆H_mix) is zero.
* Volume change of mixing (∆V_mix) is zero.
* p = p_A + p_B = p_A° x_A + p_B° x_B.
* The solute - solute, solvent - solvent interactions are same as that of solute = solute - solvent interactions.
 

8. 100 g of liquid A (molar mass 140 g mol⁻¹) was dissolved in 100 g of liquid B (molar mass 180 g mol⁻¹). The vapour pressure of pure liquid B was found to be 500 torr. Calculate the vapour pressure of pure liquid A and its vapour pressure in the solution if the total vapour pressure of the solution is 475 torr.
A: p = Total vapour pressure of the solution = 475 torr

    

2 Marks Questions: 

1. What is ppm of a solution?
A: Number of parts of one component present in one million parts of the solution.

2. State Henry's law.
A: At a given temperature, the partial pressure of a gas (p) (in vapour phase) is directly proportional to the mole fraction of the gas (x) in the solution.
               p = KH . x
       KH = Henry's law constant.
 

3. Calculate Henry's constant. If the solubility of H2 gas in water is 0.195 (at S.T.P.).
A:

 

4. Calculate the amount of Benzoic acid (C₆H₅COOH) required for preparing 250
ml of 0.15 molar solution in methanol of (CH₃OH).
A:

5. Calculate molality of 2.5 g of Ethanoic acid (CH₃COOH) in 75 g of Benzene.
A: 

      
 

6. Calculate the mole fraction of Ethylene Glycol (C₂H₆O₂) in a solution containing 20% of C₂H₆O₂ by mass.

7. Vapour pressure of water at 293 K is 17.535 mm Hg. Calculate vapour pressure of solution at 293 K when 25 g. of glucose is dissolved in 450 g of water.

∴  p = Vapour pressure of solution = 17.44 mm Hg
 

8. The vapour pressure of pure Benzene at a certain temperature is 0.850 bar. A non - volatile, non - electrolyte solid weighing 0.5 g when added to 39 g of Benzene (molar mass 78). Vapour pressure of the solution then, is 0.845 bar. What is the molar mass of the solid substance?

9. If the osmotic pressure of glucose solution is 1.52 bar at 300 K. What would be its concentration if R = 0.083 L bar mol⁻¹ K⁻¹?
A: π = Osmotic pressure of glucose = 1.52 bar
     C = Concentration of the solution = ?
     R = 0.083 L bar mol⁻¹ K⁻¹
     T = 300 K

10. 1 g of non electrolyte solution dissolved in 50 g of Benzene lowered the freezing point of Benzene by 0.40 K. The freezing point depression constant of Benzene is 5.12 K kg mol⁻¹. Find the molar mass of the solute.
A: ∆T_f = K_f .m