Class 12 Chemistry
Chapter 2 - Solutions
(Magical Series) Very Important Questions Set 7
Q.1. What mass of NaCl must be dissolved in 65.0 g of water to lower the freezing point of water by 7.50 °C? The freezing point depression constant (Kf) for water is 1.86 °C/m. Assume van't Hoff factor for NaCl is 1.87. (Molar mass of NaCl = 58.5 g mol–1).
[Ans: 8.199 g]
Q.2. Calculate the boiling point of one molar aqueous solution (density 1.06 g mol–1) of KBr.
[Given: Kb for H2O = 0.52 K kg mol–1, atomic mass: K = 39, Br = 80]
[Ans: 373.553 K]
Q.3. A solution prepared by dissolving 1.25 g of oil of winter green (methyl salicylate) in 99.0 g of benzene has a boiling point of 80.31 oc. Determine the molar mass of this compound. (B.p. of pure benzene = 80.10 °C and Kb for benzene = 2.53 °C kg mol-1)
[Ans: 152 g mol–l]
Q.4. What mass of ethylene glycol (molar mass = 62.0 g mort) must be added to 5.50 kg of water to lower the freezing point of water from 0 °C to –10.0 °C? (Kf for water 1.86 K kg mol–1).
[Ans: 1833.33 g]
Q.5. 100 mg of a protein is dissolved in just enough water to make 10.0 mL of solution. If this solution has an osmotic pressure of 13.3 mm Hg at 25°C, what is the molar mass of the protein? (R = 0.0821 L atm mol-1 K–1 and 760 mm Hg = 1 atm.).
[Ans: 13980.45 g mol–1]
Q.6. Calculate the amount of sodium chloride which must be added to one kilogram of water so that the freezing point of water is depressed by 3 K. [Given: Kf = 1.86 K kg mol–1, atomic mass: Na = 23.0, Cl = 35.5]
[Ans: 47.18 g]
Q.7. x g of a non-electrolytic compound (molar mass = 200) is dissolved in 1.0 L of 0.05 M NaCl aqueous solution. The osmotic pressure of this solution is found to be 4.92 atm at 27 °C. Calculate the value of x. Assume complete dissociation of NaCl and ideal behaviour of the solution. (R = 0.082 L atm mort K–1).
[Ans: 20 g]
Q.8. Calculate the freezing point of a solution containing 18 g glucose, C6H12O6 and 68.4 g sucrose, C12H22O11 in 200 g of water. The freezing point of pure water is 273 K and Kf for water is 1.86 K mol–1.
[Ans: 270.21 K]
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