Chemistry 12 Chapter 2 Exemplar Solutions Question and Answer Part 5

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III. Short Answer Type

Question 36:

Components of a binary mixture of two liquids A and B were being separated by distillation. After some time separation of components stopped and composition of vapour phase became same as that of liquid phase. Both the components started coming in the distillate. Explain why this happened.

Answer:

Since both the components are appearing in the distillate and composition of liquid and vapour is same, this shows that liquids have formed azeotropic mixture and hence cannot be separated at this stage by distillation.

Question 37:

Explain why on addition of of to of water, the boiling point of water increases, while addition of of methyl alcohol to one litre of water decreases its boiling point.

Answer:

is a non volatile solute, therefore, addition of to water lowers the vapour pressure of water. As a result boiling point of water increases. Methyl alcohol on the other hand is more volatile than water, therefore its addition increases, the total vapour pressure over the solution and a decrease in boiling point of water results.

Question 38:

Explain the solubility rule “like dissolves like” in terms of intermolecular forces that exist in solutions.

Answer:

A substance (solute) dissolves in a solvent if the intermolecular interactions are similar in both the components; for example, polar solutes dissolve in polar solvents and non-polar solutes in non-polar solvents thus we can say “like dissolves like”.

Question 39:

Concentration terms such as mass percentage, ppm, mole fraction and molality are independent of temperature; however molarity is a function of temperature. Explain.

Answer:

Molarity of a solution is defined as the number of moles of solute dissolved in one liter of solution. Since volume depends on temperature and undergoes a change with change in temperature, the molarity will also change with change in temperature. On the other hand, mass does not change with change in temperature, as a result other concentration terms given in the question remain unchanged by changing temperature. According to the definition of all these terms, mass of the solvent used for making the solution is related to the mass of solute.

Question 40:

What is the significance of Henry’s Law constant?

Answer:

Higher the value of Henry’s law constant, the lower is the solubility of the gas in the liquid.

Question 41:

Why are aquatic species more comfortable in cold water in comparison to warm water?

Answer:

At a given pressure the solubility of oxygen in water increases with decrease in temperature. Presence of more oxygen at lower temperature makes the aquatic species more comfortable in cold water.

Question 42:

(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?

Answer:

Refer to the NCERT textbook for Class XII.

Question 43:

Why is the vapour pressure of an aqueous solution of glucose lower than that of water?

Answer:

In pure liquid water the entire surface of liquid is occupied by the molecules of water. When a non-volatile solute, for example glucose is dissolved in water, the fraction of surface covered by the solvent molecules gets reduced because some positions are occupied by glucose molecules. As a result number of solvent molecules escaping from the surface also gets reduced; consequently the vapour pressure of aqueous solution of glucose is reduced.

Question 44:

How does sprinkling of salt help in clearing the snow covered roads in hilly areas? Explain the phenomenon involved in the process.

Answer:

When salt is spread over snow covered roads, snow starts melting from the surface because of the depression in freezing point of water and it helps in clearing the roads.

Question 45:

What is “semi permeable membrane”?

Answer:

Continuous sheets or films (natural or synthetic) which contain a network of submicroscopic holes or pores through which small solvent molecules like water can pass; but the passage of bigger molecules of solute is hindered, are known as semi permeable membrane.

Question 46:

Give an example of a material used for making semipermeable membrane for carrying out reverse osmosis.

Answer:

Cellulose acetate.