CBSE Class 12 Chemistry Solutions VBQs

Question. Ethylene glycol is used as an antifreeze in a cold climate. Mass of Ethylene glycol which should be added to 4kg water to prevent it from freezing at -60C will be (Kf for water = 1.86Kkg/mol^-1, Molar mass of Ethylene glycol = 62g/mol)
(a) 204.30g
(b) 800g
(c) 304.60g
(d) 400g
Answer. B

Question. The depression in freezing point for 1M Urea, 1M Glucose and 1M NaCl are in the ratio
(a) 1:1:2
(b) 3:2:2
(c) 1:2:3
(d) None of these
Answer. A

Question. The solution that forms maximum boiling azeotropes is
(a) Carbon disulphide – Acetone
(b) Benzene - Toluene
(c) Acetone – Chloroform
(d) n-Hexane – n-Hectane.
Answer. C

Question. The type of intermolecular interaction present in a solution of n- Hexane and n-Octane is:
(a) London dispersion forces
(b) Dipole-dipole interaction
(c) Hydrogen bonding
(d) Ion-dipole interaction
Answer. A

Question. Which among the following is least soluble in water?
(a) Phenol
(b) Toluene
(c) Ethylene glycol
(d) Pentanol
Answer. B

ASSERTION -REASON TYPE QUESTIONS

In the following questions, two statements (Assertion) A and Reason (R) are given.
(a) If A and R both are correct and R is the correct explanation of A
(b) If A and R both are correct but R is not the correct explanation of A
(c) A is true but R is false
(d) A is false but R is true

Question. Assertion: When a blood cell is placed in hypertonic solution, it shrinks.
Reason: Blood is isotonic with 0.9% NaCl solution.
Answer. B

Question. Assertion: The aquatic species feel more comfortable in winter than summer
Reason: Solubility of gases increases with increase of temperature.
Answer. C

Question. Assertion: Azeotropic mixture are formed only by non-ideal solutions.
Reason: Boiling point of an azeotropic is either higher than both the components or lower than both the components.
Answer. B

Question. Assertion: Soft drink and soda water bottles are sealed under high pressure.
Reason: The dissolution of gas in liquid is an endothermic process.
Answer. C

Question. Assertion: On adding non-volatile solute to water its vapour pressure increases.
Reason: Relative lowering of vapour pressure is a colligative property
Answer. D

Question. Assertion: Addition of ethylene glycol to water lowers the freezing point of water, therefore, used as antifreeze.
Reason: 1, 2-Ethanediol is soluble in water because it can form H-bond with water, therefore vapour pressure of solution is lowered.
Answer. A

Question. Assertion: In an ideal solution, ΔHmix is zero.
Reason: In an ideal solution, A – B interactions are lower than A-A and B-B interactions.
Answer. C

Question. Assertion: Mixture of ethanol and cyclohexane forms a solution with negative deviation from Raoult’s law.
Reason: When ethanol mixes in cyclohexane, it reduces the intermolecular force between ethanol molecules.
Answer. D

Question. Assertion: Van’t Hoff factor for Benzoic acid in Benzene is less than 1
Reason: Benzoic acid dimerizes in Benzene.
Answer. A

Question. Assertion: Larger the value of cryoscopic constant of the solvent, lesser will be the freezing point of solution.
Reason: Depression in freezing point depends on the nature of the solvent.
Answer. C

Question. Assertion: An aqueous solution of NaCl freezes below 273 K.
Reason: Vapour pressure of the solution is less than that of the pure solvent.
Answer. A

Question. Assertion: The sum of mole fractions of all components of a solution is unity.
Reason: Mole fraction is independent of temperature.
Answer. B

Question. Assertion: Osmosis involves movement of solvent molecules from its lower concentration to its higher concentration.
Reason: Solution having the same osmotic pressure are called isotonic solution.
Answer. B

Question. Assertion: When blood cell is placed in a solution containing 1.5 % NaCl, it will shrink.
Reason: Blood is isotonic with 1.5% NaCl solution.
Answer. C

Question. Assertion: Molarity of a solution in liquid state changes with temperature.
Reason: The volume of a solution changes with change in temperature.
Answer. A

SHORT ANSWER QUESTIONS

Question. Give an example of a material used for making semipermeable membrane for carrying out reverse osmosis.
Answer. Polymer cellulose acetate is used for making semipermeable membrane for carrying out reverse osmosis.

Question. Distinction between molarity and molality.
Answer. Molarity: It is the number of moles of solute dissolved in 1 litre of solution. It is temperature dependent. Molality : It is the number of moles of solute dissolved in 1 kg of the solvent and independent of temperature.

Question. Define an ideal solution and write one of its characteristics.
Answer. An ideal solution may be defined as the solution which obeys Raoult’s law exactly over the entire range of temperature and pressure. For ideal solution Heat of mixing is zero Volume change of mixing is zero.

Question. (i) Write the colligative property which is used to find the molecular mass of macromolecules. (ii) In non-ideal solution, what type of deviation shows the formation of minimum boiling azeotropes?
Answer. (i) Osmotic pressure (ii) Minimum boiling azeotropes show positive deviation from Raoult’s law.

Question. Explain why on addition of 1 mol of NaCl to 1 litre of water, the boiling point of water increases, while addition of 1 mol of methyl alcohol to one litre of water decreases its boiling point.
Answer. Sodium chloride (NaCl) is a non-volatile solute. When added to water taken in a beaker, the solute occupies some surface area. As a result, the vapour pressure decreases and the boiling point of solution increases. On the other hand, methyl alcohol is more volatile than water. The addition of methyl alcohol to water increases the total vapour pressure of the solution. The boiling point of the solution decreases.

Question. 1.00 molal aqueous solution of trichloroacetic acid (CCl₃COOH) is heated to its boiling point. The solution has the boiling point of 100.18°C. Determine the van’t Hoff factor for trichloroacetic acid. (Kb for water = 0.512 K kg mol^-1)
Answer. As ΔTb= iKbm
(100.18 – 100) °C = i × 0.512 K kg mol^-1 × 1 m
0.18 K = i × 0.512 K kg mol^-1 × 1 m
∴ i = 0.3

Question. At 300 K ,30g of glucose present per litre in its solution has an osmotic pressure of 4.98 bar.If the osmotic pressure of another glucose solution is 1.52 bar at the same temperature ,calculate the concentration of other solution
Answer. =W2RT/M2V=CRT
4.98=30XRX300/180X1=50R
1.52=CXRX300=300CR
300CR/50R=1.52/4.98=0.051M

LONG ANSWER QUESTIONS

1.a) Why is the mass determined by measuring a colligative property in case of some solutes is abnormal? Discuss it with the help of the Van’t Hoff factor. Certain solutes do not behave normally in solution in the sense that they may either undergo dissociation or association. As a result, the number of solute particles in solution changes. Since the colligative properties are linked with the number of particles, they show abnormal results. In the same way, the molecular masses of these solutes also show abnormal results. The exact behavior of the solute in solution and the extent of association or dissociation can be expressed in terms of Van’t Hoff factor (i). i = Normal/ calculated molecular mass/ Observed molecular mass. or i = Observed colligative properties/ Normal colligative properties.
• If i = 1 , solute behave normally in the solution
• If i > 1 , solute undergo dissociation in solution
• If i < 1 , solute undergo association in solution

Question. Calculate the mass of compound (molar mass = 256 g mol^-1) to be dissolved in 75 g of benzene to lower its freezing point by 0.48 K (Kf = 5.12 K kg mol^-1).
Answer. Given: ΔT_f =0.48K,W₁ =75g,M₂ =256gmol^-1 W₂ =?

Usingformula,W₂ = M₂×W₁×ΔTf1000×Kf = 256×75×0.481000×5.12 = 1.8 g

Question. a) Explain the following phenomena with the help of Henry’s law.
• (i) Painful condition known as bends.
• (ii) Feeling of weakness and discomfort in breathing at high altitude.
• (b) Why soda water bottle kept at room temperature fizzes on opening?
(i) When scuba divers go deep in the sea, solubility of atmospheric gases increases in blood. When the divers come up, there is release of dissolved gases and it leads to the formation of bubbles of nitrogen in our blood capillaries and hence there is painful sensation called bends. To avoid bends; the tanks of scuba divers are filled with He,N₂ and oxygen.
• (ii) At high altitude, partial pressure of oxygen is low, it leads to low concentration of oxygen in blood of people living there. Low concentration of oxygen develops anoxia, i.e., unable to think and act properly.
• (b) In order to increase the solubility of CO₂ gas in soft drinks and soda water, the bottles are normally sealed under high pressure. Increase in pressure increases the solubility of a gas in a solvent according to Henry’s Law. If the bottle is opened by removing the stopper or seal, the pressure on the surface of the gas will suddenly decrease. This will cause a decrease in the solubility of the gas in the liquid. As a result, it will rush out of the bottle producing a hissing noise or with a fiz. 

b )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°C. 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)
Answer.
Given : W₂ = 1.25 g, W₁ = 99 g
ΔT_b = 80.31 – 80.10°C = 0.21°C
K_b = 2.53°C kg mol-1
According to the formula :
M₂ = 1000Kb W₂ /W₁
ΔT_b Substituting these values in the formula, we get
M₂ = 1000×2.53×1.25 /99×0.21
= 152 g mol^-1

CASE BASED QUESTIONS

1. Read the following paragraph and answer the questions:
An ideal solution of two liquids is a solution in which each component obeys Raoult's law which states that the vapour pressure of any component in the solution depends on the mole fraction of that component in the solution and the vapour pressure of that component in the pure state. However, there are many solutions which do not obey Raoult's law. In other words, they show deviations from ideal behaviour which may be positive or negative. However, in either case, corresponding to a particular composition, they form a constant boiling mixtures called azeotropes.

Question. The mole fraction of Ethyl alcohol in its solution with Methyl alcohol is 0.80. The vapour pressure of pure Ethyl alcohol at this temperature is 40mm of Mercury. What is its vapour pressure in the solution if the solution is ideal?
Answer. PC₂H₅OH = xC₂H₅OH x P0C₂H₅OH
= 0.80 x 40mm
= 32 mm of Mercury

Question. Why do a solution of Phenol and Aniline exhibit negative deviation from ideal behaviour?
Answer. The Hydrogen bond formed between Phenolic proton and lone pairs of electron of Aniline is stronger than the interactions existing in pure Phenol and in pure Aniline.

Question. Write and example for maximum boiling azeotrope.
Answer. Mixture of water and Nitric acid.

Question. Why pure Ethyl alcohol cannot be obtained from rectified spirit even by fractional distillation?
Answer. Because a mixture of 95.4% alcohol and 4.6% of water forms an azeotrope

Question. When two liquids A & B are mixed the volume of the resulting solution is found to be slightly greater than sum of the volumes of A & B. Identify the type of deviation exhibited by the solution.
Answer. Positive deviation.

Question. Sunil’s friend is suffering from high blood pressure, Sunil advised him to take less quantity of common salt. As a student of chemistry why he suggested like that. What are the values associated with it?
Answer:
Higher quantity of NaCl will increase number of sodium and chloride ions in the body fluid which can increase the osmotic pressure of body fluid, i.e., blood pressure of a person. Knowledge of chemistry and applied his knowledge in daily life situation. Knowledge is useful only when put to practice and also helping mentality of child.

Question. Bharath went to his grandfather’s house in winter this year. As usual he went for fishing. His grandmother told him there will be no fishes in the lake. He noticed that it was more difficult to find fishes in winter. The fishes were deep inside the river. Whereas in summer they were on the surface and hence he was able to catch fishes.
a) Why are fishes on the surface in water than in the depth in summer?
b) What value can be derived from this?
Answer:
a) According to Henry’s law at low temperature gases are more soluble and hence as more oxygen gets dissolved in water fishes survive better even in depth of the river. In summer as the oxygen is less in water the fishes come to the surface.
b) The value that I derive from this is wisdom is superior to knowledge

Question. Ramu brought some flowers for the annual function but he observed it wilted. Raju suggested that wilted flowers revive when placed in fresh water. Why would he suggest to do like that. What value associated with it?
Answer:
Due to lack of water, flowers become wilted. When we apply fresh water , water will move into them through osmosis. Apply Knowledge in daily life situation. Problem solving ability .

Question. Scuba divers when come towards the surface, the pressure gradually decreases resulting in the release of dissolved gases leading to formation of bubbles of nitrogen gas in the blood which blocks the capillaries and thus harmful effects are created. Toavoid bends and toxic effects of high concentration of nitrogen gas, the air is diluted with helium.
After reading the above passage, answer the following questions.
(i) Which law is associated with this?
(ii) Why is the condition of bends overcome by the use of Helium
(iii) Mention the value associated with providing divers air diluted with helium.
Answer: 
(i) Henry’s Law
(ii) Solubility inversely proportional to KH.
(iii) Critical thinking and self awareness

Question. 

(a) Name the process observed when pressure on solution side is more than osmotic pressure.
(b) Write main use of this process.
(c) Mention the values associated with the above process
Answer:
(a) Reverse osmosis
(b) desalination of seawater
(c) Critical thinking and self awareness.