# Solved Exercise, Chem-11, Ch-09

### SOLUTIONS

Q.01: Choose the correct answer for the given ones.

(i) Molarity of pure water is:

(a) 1

(b) 18

(c) 55.5

(d) 6

EXPLANATION: Molarity means number of moles of solute present in liter solution. In case of pure water, one liter water is equal to 1 kg water because density of water is 1. When 1 kg or 1000 g water is divided by the molar mass of water i.e., 18 g mole-1, the answer is 55.5 moles of water. Thus, molarity of pure water is 55.5 moles/dm3.

(ii) 18 g glucose is dissolved in 90 g water. The relative lowering of vapour pressure is equal to:

(a) 1/5

(b) 5.1

(c) 1/51

(d) 6

EXPLANATION: Relative lowering of vapour pressure is equal to the mole fraction of solute (x2). The mole fraction of solute or glucose is obtained by dividing the number of moles of glucose by total number of moles. 18 g glucose is equal to 0.1 mole while 90 g water is equal to 5 mole. Thus, by dividing 0.1 by 5.1, the answer obtained is 1/51 which is equal to the lowering of vapour pressure in above example.

(iii) A solution of glucose is 10% w/v. The volume in which 1 g mole of it is dissolved will be:

(a) 1 dm3

(b) 1.8 dm3

(c) 200 cm3

(d) 900 cm3

EXPLANATION: 10% w/v solution of glucose in water means 10 g glucose is present in 100 cm3 of solution. One gram mole or 180 g of glucose will be present in how much volume of solution can be obtained by the formula: 180/10 ☓ 100 = 1800 cm3 or 1.8 dm3.

(iv) An aqueous solution of ethanol in water may have vapour pressure:

(a) Equal to that of water

(b) Equal to that of ethanol

(c) More than that of water

(d) Less than that of water

EXPLANATION: Aqueous solution of ethanol in water will make positive deviation from Raoult’s law. Thus, there will decrease in intermolecular forces and increase in volume of the solution, and the azeotropic mixture will have greater vapour pressure than that of water.

(v) An azeotropic mixture of two liquids boils at a lower temperature than either of them when:

(a) It is saturated.

(b) It shows positive deviation from Raoult’s law.

(c) It shows negative deviation from Raoult’s law.

(d) It is metastable.

EXPLANATION: When an azeotropic mixture of two liquids boils at a lower temperature than either of them, it means that mixture has greater vapour pressure than ideal solution. That is, the mixture is showing positive deviation from Raoult’s law.

(vi) In azeotropic mixture showing positive deviation from Raoult’s law, the volume of the mixture is:

(a) Slightly more than the total volume of the components.

(b) Slightly less than the total volume of the components.

(c) Equal to the total volume of the components.

(d) None of these

EXPLANATION: This is because positive deviation is shown by a liquid mixture when the intermolecular forces between the components of mixture decrease and as a result the volume of the solution increases.

(vii) Which of the following solution has the highest boiling point?

(a) 5.85% solution of sodium chloride

(b) 18.0% solution of Glucose

(c) 6.0% solution of Urea

(d) All have the same boiling point

EXPLANATION: elevation in boiling point is a colligative property which depends upon the number of solute or solvent particles. All these three solutions contain 0.1 moles of solute in 100 cm3 of solution which means they should give 0.1´NA number of particles each. Glucose and urea are covalent compounds. Their 0.1 mole will give 0.1´NA number of molecules. Whereas, NaCl is an ionic compound. Its 0.1 mole will produce 0.2´NA or double number of particles or ions. This is because one formula unit of NaCl gives two ions. Thus, due to greater number of solute particles, 5.85% solution of NaCl will cause greater elevation in boiling point.

(viii) Two solutions of NaCl and KCl are prepared separately by dissolving same amount of the solute in water. Which of the following statements is true for the solutions?

(a) KCl solution will have higher boiling point than NaCl solution.

(b) Both the solutions have different boiling point.

(c) KCl and NaCl solutions possess same vapour pressure.

(d) KCl solution possesses lower freezing point than NaCl solution.

EXPLANATION: Molar mass of NaCl is 58.5 g mole-1 and that of KCl is 74.5 g mole-1. When same amounts of these salts, say 100 g, are dissolved separately in equal amount of water, the NaCl solution will produce greater number of ions because it has greater number of moles in 100 g than KCl. Since, elevation of boiling point and depression in freezing point are colligative properties which depend upon total number of particles, therefore both these solutions will have different boiling and freezing points.

(ix) The molal boiling point constant is the ratio of the elevation in boiling point to:

(a) Molarity

(b) Molality

(c) Mole fraction of solvent

(d) Mole fraction of solute

EXPLANATION: Elevation in boiling point is directly proportional to the number of moles of solute. i.e., Tb ∝ m. The greater is the number of solute particles, the greater will be the boiling point and vice versa. Resolving the equation as: ∆Tb ∝ m OR ∆Tb= Kb ₓ m OR Kb=∆Tb/m. Thus, molal boiling point constant or Kb is the ratio between the elevation in boiling point and the molality of the solution.

(x) Colligative properties are the properties of:

(a) Dilute solutions which behave as nearly ideal solutions.

(b) Concentrated solutions which behave as nearly non-ideal solutions.

(c) Both ‘a’ & ‘b’

(d) Neither ‘a’ nor ‘b’

EXPLANATION: Colligative properties depend primarily on the concentration of solute particles in the solution, not on the nature of the solute itself. In dilute solutions, the interactions between solute particles are minimal, and each particle acts independently. This allows the colligative property to be directly related to the number of solute particles present, regardless of their type. However, as the solution becomes increasingly concentrated, the interactions between solute particles become more significant, and the colligative properties deviate from the ideal behavior predicted for dilute solutions.

Q.02: Fill in the blanks with suitable words

(i) Number of molecules of sugar in 1 dm3 of 1M sugar solution is _________. (6.02 ☓ 1023)

(ii) 100 g of a 10% aqueous solution of NaOH contains 10 g of NaOH in ________ g of water. (90)

(iii) When an azeotropic mixture is distilled, its ________ remains constant. (composition)

(iv) The molal freezing point constant is also known as ____________ constant. (cryoscopic)

(v) The boiling point of an azeotropic solution of two liquids is lower than either of them because the solution shows ___________ from Raoult’s law. (positive deviation)

(vi) Among equimolal aqueous solutions of NaCl, BaCl2 and FeCl3, the maximum depression in freezing point is shown by __________ solution. (FeCl3)

(vii) A solution of ethanol in water shows _________ deviations and gives azeotropic solution with __________ boiling point than other components. (positive, lower)

(viii) Colligative properties are used to calculate ___________ of a compound. (molar mass)

(ix) The hydration energy of Br ion is ________ than that of F ion. (smaller)

(x) The aqueous solution of NH4Cl is ________ while that of Na2SO4 is _______. (acidic, neutral)

Q.03: Indicate True or False from the given statements

(i) At a definite temperature, the amount of a solute in a given saturated solution is fixed. (TRUE)

(ii) Polar solvents readily dissolve non-polar covalent compounds. (FALSE)

CORRECTION: Non-polar solvents readily dissolve non-polar covalent compounds.

(iii) The solubility of a substance decreases with increase in temperature, if the heat of a solution is negative. (TRUE)

(iv) The rate of evaporation of a liquid is inversely proportional to the intermolecular forces of attraction. (TRUE)

(v) The molecular mass of an electrolyte determined by lowering of vapour pressure is less than the theoretical molecular mass. (TRUE)

(vi) Boiling point elevation is directly proportional to the molality of the solution and inversely proportional to boiling point of solvent. (TRUE)

(vii) All solutions containing 1g of non-volatile non-electrolyte solutes in some solvent will have the same freezing point. (FALSE)

CORRECTION: 1 g of different solutes will have different number of particles, due to difference in their molar masses. Thus, all solutions containing different solutes will have different freezing points.

(viii) The freezing point of a 0.05 molal solution of a non-volatile non-electrolyte solute in water is -0.93oC. (FALSE)

CORRECTION: The freezing point of a 0.05 molal solution of a non-volatile non-electrolyte solute in water is -0.093oC.

(ix) Hydration and hydrolysis are different process for Na2SO4. (TRUE)

(x) The hydration energy of an ion only depends upon its charge. (FALSE)

CORRECTION: The hydration energy of an ion only depends upon its charge density.