Maharashtra Board Class 12 Chemistry chapter 2 Solutions

Chapter 2 Solutions

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Question 1. Choose the most correct option.
i. The vapour pressure of a solution containing 2 moles of a solute in 2 
moles of water (vapour pressure of pure water = 24 mm Hg) is
a. 24 mm Hg
b. 32 mm Hg
c. 48 mm Hg
d. 12 mm Hg

ii. The colligative property of a solution is 
a. vapour pressure
b. boiling point
c. osmotic pressure
d. freezing point

iii. In calculating osmotic pressure the concentration of solute is expressed in
a. molarity
b. molality
c. mole fraction
d. mass percent
Answer : Molar Mass

iv. Ebullioscopic constant is the boiling 
point elevation when the concentration of solution is
a. 1m
b. 1M
c. 1 mass%
d. 1 mole fraction of solute.

v. Cryoscopic constant depends on
a. nature of solvent
b. nature of solute
c. nature of solution
d. number of solvent molecules

vi. Identify the correct statement
a. vapour pressure of solution is higher
than that of pure solvent.
b. boiling point of solvent is lower than
that of solution
c. osmotic pressure of solution is lower
than that of solvent
d. osmosis is a colligative property.

vii. A living cell contains a solution which is isotonic with 0.3 M sugar solution. What osmotic pressure develops when the cell is placed in 0.1 M KCl solution at body temperature?
a. 5.08 atm
b. 2.54 atm
c. 4.92 atm
d. 2.46 atm

viii. The osmotic pressure of blood is 7.65 atm at 310 K. An aqueous solution of 
glucose isotonic with blood has the 
percentage (by volume)
a. 5.41 %
b. 3.54 %
c. 4.53 %
d. 53.4 %

ix. Vapour pressure of a solution is
a. directly proportional to the mole 
fraction of the solute
b. inversely proportional to the mole
fraction of the solute
c. inversely proportional to the mole
fraction of the solvent
d. directly proportional to the mole
fraction of the solvent

x. Pressure cooker reduces cooking time 
for food because
a. boiling point of water involved in
cooking is increased
b. heat is more evenly distributed in the
cooking space
c. the higher pressure inside the cooker
crushes the food material
d. cooking involves chemical changes
helped by a rise in temperature.

xi. Henry’s law constant for a gas CH3
Br is 0.159 moldm-3 atm at 250°
C. What is the solubility of CH3
Br in water at 25°C and a partial pressure of 0.164 atm?
a. 0.0159 mol L-1
b. 0.164 mol L-1
c. 0.026 M
d. 0.042 M

xii. Which of the following statement is 
NOT correct for 0.1 M urea solution 
and 0.05 M sucrose solution?
a. osmotic pressure exhibited by urea
solution is higher than that exhibited
by sucrose solution
b. urea solution is hypertonic to sucrose
c. they are isotonic solutions
d. sucrose solution is hypotonic to urea

2. Answer the following in one or two 
i. What is osmotic pressure?
Answer : Osmosis is defined as the net flow or movement of solvent molecules through a semipermeable membrane.

ii. A solution concentration is expressed 
in molarity and not in molality while 
considering osmotic pressure. Why?
Answer : Molarity is the number of moles of solute per liter of solution. ... In these situations, molality is the preferred method of expressing concentration because the mass of solute and solvent in a solution does not change.

iii. Write the equation relating boiling 
point elevation to the concentration 
of solution.
Answer : The result is that in dilute ideal solutions, the extent of boiling-point elevation is directly proportional to the molal concentration (amount of substance per mass) of the solution according to the equation: ΔTb = Kb · b.

 iv. A 0.1 m solution of K2
SO4 in water has freezing point of -4.3°C. What is the value of van’t Hoff factor if Kf for water is 1.86 K kg mol-1?
Answer :

v. What is van’t Hoff factor?
Answer : It is defined as the ratio of colligative property of a solution of electrolyte divided by the collogative property of nonelectrlyte solution of the same concentration.

vi. How is van’t Hoff factor related to
degree of ionization?
Answer : For weak electrolytes, van't Hoff factor 'i' is a measure of the degree of dissociation of weak electrolyte. Higher is the value of the van't Hoff factor, higher will be the degree of dissociation. Here n is the number of ions produced by the complete dissociation of 1 mole of substance.

vii. Which of the following solutions will
have higher freezing point depression
and why ?
a. 0.1 m NaCl
b. 0.05 m Al2(SO4)3
Answer : Freezing point is directly proportional to the number of dissolved particles, it can be concluded that 0.05 M Al₂(SO₄)₃ has a higher freezing point than 0.1 M of K₃[Fe(CN)₆].

The correct answer is - 0.05 M Al₂(SO₄)₃ has a higher freezing point.

viii. State Raoult’s law for a solution containing a nonvolatile solute
Answer : Raoult's law states that the relative lowering of vapour pressure of a solution containing a nonvolatile solute is equal to the mole fraction of the solute in the solution.

ix. What is the effect on the boiling point
of water if 1 mole of methyl alcohol is added to 1 dm3
 of water? Why?
Answer : addition of 1 moe of NaCl to 1L of water, the boiling point of water increases, while addition of 1 mole of methyl alcohol to 1 L of water decreases its boilbing point

x. Which of the four colligative properties is most often used for molecular mass determination? Why?
Answer : Osmotic pressure is preferred colligative property to determine molecular mass of macromolecules. - It can be measured with precision even in dilute solutions.

3. Answer the following.
i. How vapour pressure lowering is related to a rise in boiling point of solution?
Answer : The normal boiling point is the temperature in which the vapour pressure of a liquid becomes equal to atmospheric pressure. If the intermolecular forces are small, the liquid has a high vapour pressure. Little heat energy will have to be added to separate the molecules, so the boiling point will be low.

ii. What are isotonic and hypertonic solutions?
Answer : An isotonic solution refers to two solutions having the same osmotic pressure across a semipermeable membrane. This state allows for the free movement of water across the membrane without changing the concentration of solutes on either side.

A hypertonic solution is a particular type of solution that has a greater concentration of solutes on the outside of a cell when compared with the inside of a cell.

iii. A solvent and its solution containing
a nonvolatile solute are separated by
a semipermable membrane. Does the
flow of solvent occur in both directions?
Comment giving reason.
Answer : When a solution and a pure solvent are separated by a semipermeable membrane, a barrier that allows solvent molecules but not solute molecules to pass through, the flow of solvent in opposing directions is unequal and produces an osmotic pressure, which is the difference in pressure between the two sides.

iv. The osmotic pressure of CaCl2
and ureabsolutions of the same concentration at the same temperature are respectively 0.605 atm and 0.245 atm. Calculate van’t Hoff factor for CaCl2
Answer :

v. Explain reverse osmosis.
Answer : osmosis is a flow of solvent through a semipermeable membrane into the solution. The direction of osmosis can be reversed by applying a pressure larger than the osmotic pressure.

The pure solvent then flows from
solution into pure solvent through semi-
permeable membrane. This phenomenon is
called reverse osmosis. Fig. 2.10 shows the
schematic set up for reverse osmosis. Fresh
water and salty water are separated by a
semipermeable membrane. When the pressure larger than the osmotic pressure of solution is applied to solution, pure water from salty water passes into fresh pure water through the membrane.

vi. How molar mass of a solute is
determined by osmotic pressure
Answer :
Osmotic pressure symbol= π

π∝C [Molarity]
∴π=STC where S= solution constant

∵ Molarity= (V) Volume in litres /Moles and moles= Molar mass(m)/weight(W)​

∴π=mW× V1×S.T⟶ From this equation molar mass can be determined.

vii. Why vapour pressure of a solvent is lowered by dissolving a nonvolatile
solute into it?
Answer : When a non volatile solute is added to a solvent, the vapour pressure of the resulting solution is lower than the vapour pressure of pure solvent. This is because, the surface area is partly occupied by non volatile solute molecules and partly by solvent molecules. Due to this, the rate of evaporation is lowered.

viii. Using Raoult’s law, how will you show that ∆P = 0  P1 x2? Where x2
is the mole fraction of solute in the solution and 0, P1 vapour pressure of pure solvent
Answer :

ix. While considering boiling point
elevation and freezing point depression
a solution concentration is expressed in
molality and not in molarity. Why?
Answer : Molality is the number of moles of solute per kilogram of solvent. Molarity is the number of moles of solute per liter of solution. In these situations, molality is the preferred method of expressing concentration because the mass of solute and solvent in a solution does not change.