CBSE Class 12 Chemistry Solutions Worksheet Set B

Very Short Answer Type Questions

Question: What type of intermolecular attractive interaction exists in the pair of methanol and acetone? 
Answer. Solute-solvent dipolar interactions exist in the pair of methanol and acetone.

Question: What is meant by ‘reverse osmosis’? 
Answer. If a pressure higher than the osmotic pressure is applied on the solution, the solvent will flow from the solution into the pure solvent through semipermeable membrane. This process is called reverse osmosis (R.O.).

Question: Out of BaCl₂ and KCl, which one is more effective in causing coagulation of a negatively charged colloidal Sol? Give reason. 
Answer. BaCl₂ is more effective in causing coagulation because it has double +ve charge than K⁺.

Question: What are isotonic solutions? 
Answer. An isotonic solution is a kind of solution with the same salt concentration as blood and cells. Those solutions which are exerting same osmotic pressure under similar conditions (For example 0.9% NaCl solution by mass volume is Isotonic with human blood).

Question: What type of semiconductor is obtained when silicon is doped with arsenic?
Answer. n–type semiconductor.

Question: Some liquids on mixing form ‘azeotropes’. What are ‘azeotropes’? 
Answer. The liquid mixture having a definite composition and boiling like a pure liquid without change in composition is called as azeotrope.

Short Answer Type Questions-I

Question: State Henry’s law and mention two of its important applications. 
Answer. Henry’s law : Henry’s law states that “The partial pressure of the gas in vapour phase is proportional to the mole fraction of the gas in the solution”.
Applications of Henry’s law :
(i) To increase the solubility of CO₂ in soft drinks and soda water, the bottle is sealed under high pressure.
(ii) To avoid a dangerous medical condition called bends, scuba divers use oxygen diluted with less soluble helium gas. 

Question: Define the terms, ‘osmosis’ and ‘osmotic pressure’.What is the advantage of using osmotic pressure as compared to other colligative properties for the determination of molar masses of solutes in solutions?
Answer. Osmosis : The net spontaneous flow of the solvent molecules from the solvent to the solution or from a less concentrated solution to a more concentrated solution through a semipermeable membrane is called osmosis.
Osmotic pressure : The minimum excess pressure that has to be applied on the solution to prevent the entry of the solvent into the solution through the semipermeable membrane is called the osmotic pressure.
The osmotic pressure method has the advantage that it uses molarities instead of molalities and it can be measured at room temperature.

Question: Explain why aquatic species are more comfortable in cold water rather than in warm water. 
Answer. Aquatic species need dissolved oxygen for breathing. As solubility of gases decreases with increase of temperature, less oxygen is available in summer in the lake. Hence the aquatic species feel more comfortable in winter (low temperature) when the solubility of oxygen is higher.

 Question: Why do gases nearly always tend to be less soluble in liquids as the temperature is raised?
Answer. This is because the dissolution of gas in liquid is an exothermic process. The solubility should decrease with increase in temperature.

 Question: Define osmotic pressure. How is osmotic pressure related to the concentration of a solute in a solution? 
Answer. Osmotic pressure is the measure of excess pressure applied on solution side to stop the process of osmosis. Osmotic pressure is directly proportional to the conentration of solute in solution π ∝ C

Question: (i) Gas (A) is more soluble in water than Gas
(B) at the same temperature. Which one of the two gases will have the higher value of KH (Henry’s constant) and why?
(ii) In non-ideal solution, what type of deviation shows the formation of maximum boiling azeotropes? 
Answer. (i) Gas (B) will have higher value of K_H
(Henry’s constant) than Gas (A) at the same temperature because lesser the solubility of a gas in a given solvent, higher will be the value of KH for a gas.
K_H = Partial pressure of gas/Mole fraction of gas in the solution
(ii) Negative deviations from Raoult’s law show the formation of maximum boiling azeotropes.

Question: A 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 = 0.35

Question: How is the vapour pressure of a solvent affected when a non-volatile solute is dissolved in it?
Answer. The vapour pressure of a solvent decreases when a non-volatile solute is dissolved in it because some solvent molecules are replaced by the molecules of solute.

Question: Define the following terms:
(i) Abnormal molar mass
(ii) van’t Hoff factor (i) 
Answer. (i) Abnormal molar mass. If the molar mass calculated by using any of colligative properties tends to be different than theoretically expected molar mass, it is called abnormal molar mass.
(ii) van’t Hoff factor (i). Extent of dissociation or association or ratio of the observed colligative property to calculated colligative property.
i = Observed colligative property/Theoretical colligative property

Question:Define azeotropes. What type of azeotrope is formed by positive deviation from Raoult’s law? Given an example. 
Answer. Azeotropes : Liquid mixture which distills without change in compositions are called azeotropic mixtures or Azeotropes.
In positive deviations from Raoult’s law,minimum boiling point azeotropic mixture is formed. For example, 95% ethanol + 5% water.

Question: What is meant by positive deviations from Raoult’s law? Give an example. What is the sign of ΔmixH for positive deviation? 
Answer. In positive deviations, the partial vapour pressure of each component A and B of a solution and the total pressure of the solution is higher than the vapour pressure calculated from Raoult’s law.
For example, Water and Ethanol.
In case of positive deviations, Δ_mixH > 0 (Positive)

Question: What is osmotic pressure? Why it is a colligative property? (Comptt. Delhi)
Answer. The excess pressure applied on solution side to stop the process of osmosis. Because it depends upon the number of solute particles but not on their nature.

Question: (i) On mixing liquid X and liquid Y, volume of the resulting solution decreases. What type of deviation from Raoult’s law is shown by the resulting solution? What change in temperature would you observe after mixing liquids X and Y?
(ii) What happens when we place the blood cell in water (hypotonic solution)? Give reason.
Answer. (i) Volume decreases by mixing X and Y. It
shows negative deviations from Raoult’s law. There will be rise in temperature.
(ΔH_mix < 0)
(ii) Blood cell will swell due to osmosis as water enters the cell.

Question: Define osmotic pressure of a solution. How is the osmotic pressure related to the concentration of a solute in a solution? 
Answer. Osmotic pressure : It is the external pressure which is applied on the side solution which is sufficient to prevent the entry of the solvent through semi-permeable membrane.
According to the Boyle-van’t Hoff Law, the osmotic pressure (π) of a dilute solution is directly proportional to its molar concentration provided temperature is constant.
π ∝ C (At constant temperature)
π ∝ CT (At constant concentration)
π = CRT (R = Solution constant)
or, π = (n/v) RT

Question: Define an ideal solution and write one of its characteristics. 
Answer. Those solutions which are obeying Raoult’s law are called ideal solutions. An ideal solution is a solution in which no volume change and no enthalpy change takes place on mixing the solute and the solvent in any proportion.
Characteristic of an ideal solution :
There will be no change in enthalpy ΔH_mixing = 0,
ΔV_mix = 0, ΔP_mix = 0

Question: State Henry’s law. What is the effect of temperature on the solubility of a gas in a liquid? 
Answer. Henry’s law : Henry’s law states that, “The solubility of a gas in a liquid at a particular temperature is directly proportional to the pressure of the gas in equilibrium with the liquid at that temperature.”
Solubility of gas decreases with increase of temperature at the same pressure.

Question: Define the following terms:
(i) Colligative properties
(ii) Molality (m)
Answer. (i) Colligative properties. All those properties which depend on the number of solute particles irrespective of the nature of solute are called as colligative properties.
(ii) Molality (m). Number of moles of solute dissolved per kg of the solvent.

Question: State Raoult’s law for the solution containing volatile components. What is the similarity between Raoult’s law and Henry’s law?
Answer. Raoult’s law : “In a solution, the vapour pressure of a component at a given temperature is equal to the mole fraction of that component in the solution multiplied by the vapour pressure of that component in pure state.”
Similarity between Raoult’s law and Henry’s law is that the partial pressure or vapour pressure of the volatile component (gas) is directly proportional to the mole fraction of that component in the solution.

Long Answer Type Questions 

Question: (a) Explain the following :
(i) Henry’s law about dissolution of a gas in a liquid
(ii) Boiling point elevation constant for a solvent
(b) A solution of glycerol (C₃H₈O₃) in water was prepared by dissolving some glycerol in 500 g of water. This solution has a boiling point of 100.42°C. What mass of glycerol was dissolved to make this solution?
(Kb for water = 0.512 K kg mol^–1)
Answer. (a) (i) Henry’s law : The law states “that at a constant temperature, the solubility of a gas in a liquid is directly proportional to the pressure of the gas.”
(ii) Boiling point elevation constant for a solvent or molal elevation constant may be defined as the elevation in the boiling point when the molality of the solution is unity.

Question: (a) A 10% solution (by mass) of sucrose in water has a freezing point of 269.15 K.Calculate the freezing point of 10% glucose in water if the freezing point of pure water is 273.15 K.
Given: (Molar mass of sucrose = 342 g mol^–1)
(Molar mass of glucose = 180 g mol^–1)
(b) Define the following terms:
(i) Molality (m)
(ii) Abnormal molar mass (All India)
Answer. 
(b) (i) Molality (m). Number of moles of solute dissolved per kg of the solvent.
(ii) Abnormal molar mass. If the molar mass calculated by using any of the colligative properties comes to be different than theoretically expected molar mass.