Class 12 Chemistry
Chapter 2 - Solutions
(Magical Series) Very Important Questions Set 9
Q.1. A 0.561 m solution of an unknown electrolyte depresses the freezing point of water by 2.93 °C. What is van't Hoff factor for this electrolyte? The freezing point depression constant (Kf) for water is 1.86 °C kg mol–1.
[Ans: 2.807]
Q.2. Phenol associates in benzene to a certain extent to form a dimer. A solution containing 20 g of phenol in 1.0 kg of benzene has its freezing point lowered by 0.69 K. Calculate the fraction of phenol that has dimerised [Given Kf for benzene= 5.1 K mol–1]
[Ans: 73 %]
Q.3. An aqueous solution containing 12.48 g of barium chloride in 1.0 kg of water boils at 373.0832 K. Calculate the degree of dissociation of barium chloride. [Given Kb for H2O = 0.52 K mol–1; Molar mass of BaCl2 = 208.34 g mol–1].
[Ans: 83.5 %]
Q.4. A decimolar solution of potassium ferrocyanide K4[Fe(CN)6] is 50% dissociated at 300 K. Calculate the value of van't Hoff factor for potassium ferrocyanide. Also calculate the osmotic pressure.
[Ans: 3, 7.389 atm]
Q.5. Calculate the boiling point of solution when 4 g of MgSO4 (M = 120 g mol–1) was dissolved in 100 g of water, assuming MgSO4 undergoes complete ionization. (Kb for water = 0.52 K kg mol–1).
[Ans: 100.34 °C]
Q.6. 3.9 g of benzoic acid dissolved in 49 g of benzene shows a depression in freezing point of 1.62 K. Calculate the van't Hoff factor and predict the nature of solute (associated or dissociated). (Given: Molar mass of benzoic acid = 122 gm, Kf for benzene = 4.9 K kg mol–1).
[Ans: 0.506, associated]
Q.7. Calculate the mass of NaCl (molar = 58.5 g mol–1) to be dissolved in 37.2 g of water to lower the freezing point by 2 °C, assuming that NaCl undergoes complete dissociation. (Kf for water = 1.86 K kg m or 1).
[Ans: 1.17 g]
Q.8. Determine the osmotic pressure of a solution prepared by dissolving 2.5 × 10–2 g of K2SO4 in 2 L of water at 25°C, assuming that it is completely dissociated. (R = 0.0821 L atm K–1 mol–1, molar mass of K2SO4 = 174 g mol–1).
[Ans: 0.1758 ×10–2 atm]
Q.9. Calculate the amount of KCl which must be added to 1 kg of water so that the freezing point is depressed by 2 K (the Kf for water = 1.86 K kg mol–1).
[Ans: 40.05 g]
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