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Practice each question, then open the answer or worked solution.
For a binary ideal liquid solution, the variation in total vapour pressure versus
the composition of the solution is given by which of the curves?
Option (i) and (iv) are the answers.
Colligative properties are observed when _____________.
(i) a non-volatile solid is dissolved in a volatile liquid.
(ii) a non-volatile liquid is dissolved in another volatile liquid.
(iii) a gas is dissolved in non-volatile liquid.
(iv) a volatile liquid is dissolved in another volatile liquid.
Option (i) and (ii) are the answers.
III. Short Answer Type
Components of a binary mixture of two liquids A and B were being separated by distillation. After some time separation of components stopped and the composition of the vapour phase became the same as that of a liquid phase. Both the components started coming in the distillate. Explain why this happened.
Both the components started coming in the distillate and the vapour and liquid phase will have the same composition. This shows an azeotropic mixture. Therefore it can’t be separated by distillation.
Explain why on the addition of 1 mol of NaCl to 1 litre of water, the boiling point of water increases, while the addition of 1 mol of methyl alcohol to one litre of water decreases its boiling point.
On addition of 1 mol of NaCl to 1 litre of water, the boiling point of water increases, while the addition of 1 mol of methyl alcohol to one litre of water decreases its boiling point because if a non-volatile solute like NaCl is added this happens. Methyl alcohol is volatile than water so it decreases the boiling point.
Explain the solubility rule “like dissolves like” in terms of intermolecular forces that exist in solutions.
“Like dissolves like” means that a solute dissolves in a solvent if their intermolecular interactions are similar. That is if polar solutes can dissolve in polar solvents and non-polar solutes can dissolve in non-polar solvents.
Concentration terms such as mass percentage, ppm, mole fraction and molality are independent of temperature, however, molarity is a function of temperature. Explain.
A change in temperature causes a change in volume, therefore changes the molarity of a solution. Hence, terms such as mass percentage, ppm, mole fraction, and molality are independent of temperature as they are calculated based on mass and do not involve volume.
What is the significance of Henry’s Law constant KH?
Henry’s law states that in a solution, “the partial pressure of the gas in the vapour phase (p) is proportional to the mole fraction of the gas (x) in the solution”
The equation is given as p = KHx, where KH is the Henry Law’s constant. The constant is a function of the nature of the gas. If kH value is high at a given temperature, then lower will be the solubility of a gas in the liquid.
Why are aquatic species more comfortable in cold water in comparison to warm water?
Aquatic species are more comfortable in cold water due to the increased solubility and availability of O2.
(a) Explain the following phenomena with the help of Henry’s law.
(i) A 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) Bends is a condition that occurs to sea drivers or scuba drivers. They have to breathe air under high pressure underwater. Nitrogen gas is not soluble at normal pressure. In underwaters, they dissolve in the blood. Once the divers come up to the surface, the gases escape the blood by forming bubbles of N2 in blood. These bubbles can block capillaries and obstruct the flow of O2, which forms the painful condition of bends.
(ii) At higher altitudes, atmospheric pressure is low and it reduces the solubility of oxygen in blood and tissues of people travelling or living at high altitudes. The lack of oxygen causes weakness and unable to think properly, a condition known as anoxia.
(iii) When a soda water bottle is opened at room temperature and normal pressure conditions, the pressure inside the bottle reduces and solubility of CO2 reduces, thus causing the gas bubbles to escape and soda water fizzing.
Why is the vapour pressure of an aqueous solution of glucose lower than that of water?
Example of water and glucose can be taken for explaining this. In water, the surface of the liquid will be occupied by the water molecules. When we add glucose into it, some of the surfaces will be occupied by the glucose molecules. Therefore the number of a solvent molecule will escape from the surface and thus vapour pressure also get reduced.