25. Which of the following statements is(are) true? Correct the false statements
ID: 704404 • Letter: 2
Question
25. Which of the following statements is(are) true? Correct the false statements. a. The vapor pressure of a solution is directly related to the mole fraction of solute. b. When a solute is added to water, the water in solution has a lower vapor pressure than that of pure ice at 0°C. c. Colligative properties depend only on the identity of the solute and not on the number of solute particles present. d. When sugar is added to water, the boiling point of the solution increases above 100°C because sugar has a higher boiling point than water.Explanation / Answer
25) (a) The vapor pressure of a solution (P) is related to the mole fraction of the solvent in the solution (?solv) by the relation
P = ?solv*P0
where P0 is the vapor pressure of the pure solvent.
Therefore, the vapor pressure of the solution is directly proportional to the mole fraction of the solvent and hence, the given statement is false.
(b) When a solute is added to water, the vapor pressure of the solution (P) is given by Raoult’s law
P = ?solv*P0
where ?solv is the mole fraction of water in the solution and P0 is the vapor pressure of pure ice at 0°C. Since a solution is composed of a solute and a solvent, hence, the mole fraction of the solvent ?solv < 1, i.e, P is less than P0. Thus, the given statement is true.
(c) Colligative properties of a solution, like vapor pressure, freezing point depression, boiling point elevation, etc. are defined as properties of the solution which depend on the ratio of the number of solute and solvent particles, i.e, colligative properties are dependent on the number of both solute and solvent particles. The identity of solute and solvent particles do not play any role in deciding colligative properties. Thus, the same number of glucose or sucrose molecules dissolved in the same number of water molecules will produce the same vapor pressure or freezing point depression. Thus, the given statement is false.
(d) When sugar is dissolved in water, the boiling point of the solution rises. However, this is not because the boiling point of sugar is higher than that of sugar; rather, this is due to a phenomenon called colligative properties of solutions. Therefore, the given statement is false.
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