12th Standard CBSE Chemistry Case study Questions

CBSE 12th Standard Chemistry Subject Solution Chapter Case Study Questions 2021

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CBSE 12th Standard Chemistry Subject Solution Case Study Questions 2021

12th Standard CBSE

Chemistry

Time : 00:30:00 Hrs

Total Marks : 20

Read the passage given below and answer the following questions:
The solubility of gases increases with increase of pressure. William Henry made a systematic investigation of the solubility of a gas in a liquid. According to Henry's law "the mass of a gas dissolved per unit volume of the solvent at constant temperature is directly proportional to the pressure of the gas in equilibrium with the solution".
Dalton during the same period also concluded independently that the solubility of a gas in a liquid solution depends upon the partial pressure of the gas. If we use the mole fraction of gas in the solution as a measure of its solubility, then Henry's law can be modified as "the partial pressure of the gas in the vapour phase is directly proportional to the mole fraction of the gas in the solution":
(i) Henry's law constant for the solubility of methane in benzene at 298 K is 4.27 X 10⁵ mm Hg. The solubility of methane in benzene at 298 K under 760 mm Hg is

(a) 4.27 x 10^-5	(b) 1.78 x 10^-3
(c) 4.27 X 10^-3	(d) 1.78 x 10^-5

(ii) The partial pressure of ethane over a saturated solution containing 6.56 x 10^-2 g of ethane is 1 bar. If the solution contains 5.00 x 10^-2 g of ethane then what will be the partial pressure (in bar) of the gas?

(a) 0.762

(b) 1.312

(c) 3.81

(d) 5.0

(iii) K_H (K bar) values for Ar_(g), CO_2(g), HCHO_(g)and CH_4(g) are 40.39, l.67, 1.83 x 10^-5 and 0.413 respectively.
Arrange these gases in the order of their increasing solubility.

\(\text {(a) } \mathrm{HCHO}<\mathrm{CH}_{4}<\mathrm{CO}_{2}<\mathrm{Ar}\)	\(\text { (b) } \mathrm{HCHO}<\mathrm{CO}_{2}<\mathrm{CH}_{4}<\mathrm{Ar}\)
\(\text {(c) } \mathrm{Ar}<\mathrm{CO}_{2}<\mathrm{CH}_{4}<\mathrm{HCHO}\)	\(\text { (d) } \mathrm{Ar}<\mathrm{CH}_{4}<\mathrm{CO}_{2}<\mathrm{HCHO}\)

(iv) Which of the following statements is correct

(a) K_H increases with increase of temperature

(b) K_H decreases with increase of temperature

(c) K_H remains constant with increase oftemperature

(d) K_H first increases then decreases, with increase of temperature.

Read the passage given below and answer the following questions:
At the freezing point of a solvent, the solid and the liquid are in equilibrium. Therefore, a solution will freeze when its vapour pressure becomes equal to the vapour pressure of the pure solid solvent.
It has been observed that when a non-volatile solute is added to a solvent, the freezing point of the solution is
always lower than that of the pure solvent. Depression in freezing point can be given as, \(\Delta T_{f}=K_{f} m\)
Where, K_f = Molal freezing point depression constant
or we can write, \(\Delta T_{f}=\frac{K_{f} \times W_{B} \times 1000}{W_{A}^{1} \times M_{B}}\)
In these questions (i-iv), a statement of assertion followed by a statement of reason is given. Choose the correct answer out of the following choices.
(a) Assertion and reason both are correct statements and reason is correct explanation for assertion.
(b) Assertion and reason both are correct statements but reason is not correct explanation for assertion.
(c) Assertion is correct statement but reason is wrong statement.
(d) Assertion is wrong statement but reason is correct statement.
(i) Assertion: 0.1 M solution of glucose has same depression in the freezing point as 0.1 M solution of urea.
Reason: K_ffor both has same value.
(ii) Assertion: Larger the value of cryoscopic constant of the solvent, lesser will be the freezing point of the solution.
Reason: Extent of depression in the freezing point depends on the nature of the solvent.
(iii) Assertion: The water pouch of instant cold pack for treating athletic injuries breaks when squeezed and NH₄NO₃ dissolves thus lowering the temperature.
Reason: Addition of non-volatile solute into solvent results into depression of freezing point of solvent.
(iv) Assertion: If a non-volatile solute is mixed in a solution then elevation in boiling point and depression in freezing point both will be same.
Reason: Elevation in boiling point and depression in freezing point both depend on number of particles of solute.

Read the passage given below and answer the following questions:
According to Raoult's law, the partial pressure of two components of the solution may be given as:
\(p_{A}=p_{A}^{\circ} x_{A} \text { and } p_{B}=p_{B}^{\circ} x_{B}\)
For an ideal solution (obeys Raoult's law always)
\(\Delta H_{\operatorname{mix}}=0, \Delta V_{\operatorname{mix}}=0\)
All solutions do not obey Raoult's law over entire range of concentration. These are known as non-ideal solutions.
For non-ideal solutions, \(p_{A} \neq p_{A}^{\circ} x_{A} \text { or } p_{B} \neq p_{B}^{\circ} x_{B}\)
Positive deviation \(\Rightarrow p_{A}>p_{A}^{\circ} x_{A} \text { and } p_{B}>p_{A}^{\circ} x_{B}\)
Negative deviation \(\Rightarrow p_{A}<p_{A}^{\circ} x_{A} \text { and } p_{B}<p_{B}^{\circ} x_{B}\)
In these questions (i-iv), a statement of assertion followed by a statement of reason is given. Choose the correct answer out of the following choices.
(a) Assertion and reason both are correct statements and reason is correct explanation for assertion.
(b) Assertion and reason both are correct statements but reason is not correct explanation for assertion.
(c) Assertion is correct statement but reason is wrong statement.
(d) Assertion is wrong statemeht but reason is correct statement.
(i) Assertion: An ideal solution obeys Raoults law.
Regson : In an ideal solution, solute-solute as well as solvent-solvent interactions are similar to solutesolvent interactions.
(ii) Assertion: Acetone and aniline show negative deviations.
Reason: H-bonding between acetone and aniline is stronger than that between acetone-acetone and aniline-aniline.
(iii) Assertion: The solutions which show negative deviations from Raoult's law are called maximum boiling azeotropes.
Reason: 68% nitric acid and 32% water by mass form maximum boiling azeotrope.
(iv) Assertion: \(\Delta H_{\mathrm{mix}}\) and \(\Delta V_{\operatorname{mix}}\)are positive for an ideal solution.
Reason: The interactions between the particles of the components of an ideal solution are almost identical as between particles in the liquids.

Read the passage given below and answer the following questions:
The phenomenon of the flow of solvent through a semipermeable membrane from pure solvent to the solution is called osmosis.
Sometimes a pressure is applied to stop the process of osmosis, this is known as osmotic pressure. It is denoted by \(\pi \). Osmotic pressure is expressed as: \(\pi \) = CRT
Since, osmotic pressure depends upon the molar concentration of solution, therefore it is a colligative property.
In these questions (i-iv), a statement of assertion followed by a statement of reason is given. Choose the correct answer out of the following choices.
(a) Assertion and reason both are correct statements and reason is correct explanation for assertion.
(b) Assertion and reason both are correct statements but reason is not correct explanation for assertion.
(c) Assertion is correct statement but reason is wrong statement.
(d) Assertion is wrong statement but reason is correct statement.
(i) Assertion: If red blood cells were removed from the body and placed in pure water, pressure inside the cells increases.
Reason: The concentration of salt content in the cells increases.
(ii) Assertion: When a solution is separated from the pure solvent by a semipermeable membrane, the solvent molecules pass through it from pure solvent side to the solution side.
Reason: Diffusion of solvent occurs from a region of high concentration to a region of low concentration solution.
(iii) Assertion: Two solutions having same osmotic pressure at a given temperature are called isotonic solutions.
Reason: Osmotic pressure is not a colligative property.
(iv) Assertion: The preservation of meat by salting and fruits by adding sugar protects against bacterial action.
Reason: A bacterium on salted meat or candid fruit loses water due to osmosis shrivels and ultimately dies.

Read the, passage given below and answer the following questions:
If some solute is added to a solvent, the boiling point of solution increases. This is known as elevation in boiling point.
\(\Delta T_{b}=K_{b} m\) ,where, K_b = Molal elevation constant
\(\Delta T_{b} \propto m\)
Hence, it is a colligative property.
Also, \(K_{b}=\frac{M R T_{b}^{2}}{\Delta_{\text {vap }} H \times 1000}\)
where, M= Molar mass of solvent
\(\Delta_{\text {vap }} H\) = Enthalpy of vaporisation
Molar mass can also be calculated using elevation in boiling point.
\(M_{B}=\frac{K_{b} \times W_{B} \times 1000}{\Delta T_{b} \times W_{A}}\)
In these questions (i-iv), a statement of assertion followed by a statement of reason is given. Choose the correct answer out of the following choices.
(a) Assertion and reason both are correct statements and reason is correct explanation for assertion.
b) Assertion and reason both are correct statements but reason is not correct explanation for assertion.
(c) Assertion is correct statement but reason is wrong statement.
(d) Assertion is wrong statement but reason is correct statement.
(i) Assertion: In a pressure cooker, the water is brought to boil. The cooker is then removed from the stove. Now on removing the lid of pressure cooker, the water starts boiling again.
Reason: The impurities in water bring down its boiling point.
(ii) Assertion: On dissolving 3.24 g of sulphur in 40 g of benzene, boiling point of solution get higher than that of benzene by 0.081 K, then the formula of sulphur is S₈. (K_b for benzene = 2.53 K kg mol
Reason: Molecular mass of sulphur comes out to be 253.
(iii) Assertion: When sugar is added to water, boiling point of water increases.
Reason: When a non-volatile solute is added to a solvent, elevation in boiling point is observed.
(iv) Assertion: Cooking time in pressure cookers is reduced.
Reason: Boiling point inside the pressure cooker in raised.

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CBSE 12th Standard Chemistry Subject Solution Case Study Questions 2021 Answer Keys

(i) (b): K_H = 4.27 x 10⁵ mm Hg
P = 760mm Hg
According to Henry's law, \(p=K_{\mathrm{H}} \times x_{\mathrm{CH}_{4}}\)
\(x_{\mathrm{CH}_{4}}=\frac{p}{K_{\mathrm{H}}}=\frac{760}{4.27 \times 10^{5}}=1.78\ \times 10^{-3}\)
(ii) (a): According to Henry's law, m = K_H x P
6.56 X 10^-2 = K_H X 1
K_H = 6.56 X 10^-2
For another case,5x 10^-2 = 6.56 X 10^-2 x P
\(p=\frac{5 \times 10^{-2}}{6.56 \times 10^{-2}}=0.762 \mathrm{bar}\)
(iii) (c) : Higher the value of K_H at a given pressure, the lower is the solubility of the gas.
(iv) (a)

(i) (b): Depression in freezing point is a colligative property which depends on the number of particles present in the solution. As both 0.1 M solution of glucose and 0.1 M solution of urea contain same number of moles (number of particles) therefore, both will have same depression in freezing point.
(ii) (a)
(iii) (a): Freezing point of a substance is defined as the temperature at which the vapour pressure of its liquid is equal to the vapour pressure of the corresponding solid. Since the addition of a non-volatile solute always lowers the vapour pressure of solvent, therefore it will be in equilibrium with solid phase at a lower pressure and hence at a lower temperature.
(iv) (d): Elevation in boiling point \(\left(\Delta T_{b}\right)=K_{b} \times m\)
Depression in freezing point \(\left(\Delta T_{f}\right)=K_{f} \times m\)
Elevation in boiling point and depression in freezing point are colligative properties i.e., they depend only on the number of particles of the solute. Value of K_band K_f are different, so \(\Delta T_{b}\) and \(\Delta T_{f}\) are also different.

(i) (a)
(ii) (a)
(iii) (b)
(iv) (d): For ideal solution \(\Delta H_{\operatorname{mix}}=0, \Delta V_{\operatorname{mix}}=0\)

(i) (c) :If the red blood cells are placed in pure water, pressure inside the cells increases as the water is drawn in and the cell swells.
(ii) (b)
(iii) (c) : Osmotic pressure is a colligative property
(iv) (a)

(i) (c): In pressure cooker, water boils above 100°C. When the lid of cooker is opened, pressure is lowered so that boiling point decreases and water boils again.
(ii) (a) : \(M_{B}=\frac{K_{b} \times 1000 \times W_{B}}{\Delta T_{b} \times W_{A}}\)
\(K_{b}=2.53 \mathrm{~K} \mathrm{~kg} \mathrm{~mol}^{-1}, W_{B}=3.24 \mathrm{~g}\)
\(\Delta T_{b}=0.81 \mathrm{~K}, W_{A}=40 \mathrm{~g}\)
\(M_{B}=\frac{2.53 \times 1000 \times 3.24}{0.81 \times 40}=253\)
Let molecular formula of sulphur = S_x
\(x \times 32=253 \text { or } x=7.91 \approx 8\)
(iii) (a)
(iv) (a)

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