Free Solutions and Colloids MCQs with Answers

41. Azeotropic mixture of water and HCl boils at 381.5 K. By distilling the mixture it is possible to obtain

• A. Pure HCl only
• B. Pure water only
• C. Neither HCl nor water
• D. Both water and HCl in pure state

Explanation: An azeotrope is a mixture that exhibits the same concentration in the vapor phase and the liquid phase. This is in contrast to ideal solutions with one component typically more volatile than the other; this is how we use distillation to separate materials. If the mixture forms an azeotrope, the vapor and the liquid concentrations are the same, which preventing separation via this approach.

42. The relative lowering of vapour pressure in case of very dilute solutions is directly proportional to:

• A. Molality
• B. Molarity
• C. Mole fraction
• D. All of these

Explanation: In case of dilute solution M=m∝ mole fraction.

43. The substances whose solubility decreases with increase in temperatures:

• A. Ca(OH)2
• B. Na2CO3
• C. Na2SO4
• D. All of these

Explanation: All get dissolved with evolution of heat.

44. On mixing 10 mL of carbon tetrachloride with 10 mL of benzene the total volume of the solution is:

• A. > 20 mL
• B. < 20 mL
• C. = 20 mL
• D. Cannot be predicted

Explanation: No changes in interparticle forces as both are non-polar.

45. The vapour pressure of pure benzene at 50 C° is 268 torr. How many mol of non-volatile solute per mol of benzene is required to prepare a solution of benzene having a vapour pressure of 167 torr at 50 C° .

• A. 0.377
• B. 0.605
• C. 0.623
• D. 0.395

Explanation: By Raoult's law x_benzene = P/P° = 167/268 = 0.623, so x_solute = 0.377. Moles of solute per mole of benzene = x_solute / x_benzene = 0.377 / 0.623 ≈ 0.605.

46. Which one of the following salts would have the same value of the Vant Hoff factor (i) as that of K3[Fe(CN)6].

• A. NaCl
• B. Na2SO4
• C. Al2(SO4)3
• D. Al(NO3)3

Explanation: K3[Fe(CN)6]dissociates into 3K+ ions and one [Fe(CN)6]3- ion. So its Van't Hoff factor is 4. Al(NO3)3 dissocaites into one Al3+ and three NO3- ions

47. The particles of a particular colloidal solution of arsenic trisulphide (As2S3) are negatively charged. Which 0.0005M solution of the following salts would be most effective in coagulating this colloidal solution?

• A. KCl
• B. MgCl2
• C. AlCl3
• D. Na3PO4

Explanation: Since As2S3 is a negatively charged colloidal sol, hence positively charged ion will cause its coagulation. By Hardy−Schulze rule "greater the charge on ion, greater the coagulating power to coagulate oppositely charged colloidal sol", hence out of K+ , Mg2+ , Al3+ and Na+ , Al3+ would be most effective in coagulating this colloidal solution.

48. Molecules at the surface of solid have.................energy than molecules those inside.

• A. higher
• B. lower
• C. some times higher & some times lower
• D. none

Explanation: At the surface, a molecule is on one side surrounded by molecules of the same type and on the other side of another type. Therefore the net interaction force is no longer zero. So molecules at the surface of the solid have more energy than the molecules inside the solid.

49. The adsorption of H2 on glass takes place on H2 (g) → 2H for this adsorption:

• A. ∆H>0, ∆S<0
• B. ∆H<0, ∆S> 0
• C. ∆H<0 , ∆S< 0
• D. ∆ H>0,∆ S<0

Explanation: Both ∆H & ∆S are positive, while ∆G<0. which makes the adsorption process feasible.

50. Mainly which colligative property is used to determine molar mass of a polymer?

• A. elevation in boiling point
• B. depression in freezing point
• C. osmotic pressure
• D. relative lowering of vapour pressure

Explanation: For determining the molar mass of a polymer osmotic pressure is preferred because it is done at normal (room) temperature. At higher temperatures molecules are not stable.