This set of Chemistry Multiple Choice Questions & Answers (MCQs) focuses on “Vapour Pressure of Liquid Solutions”.
1. What phenomenon occurs when a solution’s equilibrium vapor pressure equals the surrounding atmospheric pressure?
Explanation: Boiling, by definition, occurs when the vapor pressure of a liquid equals the surrounding atmospheric pressure. It is a very quick vaporization process which takes place at a constant temperature, referred to as the boiling point.
2. What is the boiling point of water?
a) 100 °F
b) 671.67 °R
c) 373 °C
d) 212 °r
Explanation: °R represents the temperature measured on Rankine scale. Typically, boiling point of water is measured as 100 °C on the Celsius scale. The conversion of Celsius to Rankine scale is R = C x 9/5 + 491.67.
Considering boiling point of water as 100 °C, R = 100 x 9/5 + 491.67 = 671.67 °R
°F, °r represent the Fahrenheit and Reaumur scale, respectively.
3. “Total pressure of gas mixture is the sum of individual pressures”. Which law is reflected in this statement?
a) Amagat’s law
b) Raoult’s law
c) Dalton’s law
d) Henry’s law
Explanation: Dalton’s of partial pressure states that for a non-reactive mixture of gases in a closed vessel, the total pressure of the mixture is the sum of pressures all individual gas components present. If gases A, B and C are present then, total pressure, Ptotal = PA + PB + PC.
4. If a mixture of A and B boils at a temperature lower than the boiling point of either of the components, what kind of deviation does the mixture show?
a) No deviation
b) Maximum and minimum deviation from Raoult’s law
c) Negative deviation from Raoult’s law
d) Positive deviation from Raoult’s law
Explanation: Deviation is always exhibited by non-solutions. Non- ideal solutions do not obey Raoult’s behavior. This is because ΔHmix, ΔVmix ≠ 0. The cause for this is the intermolecular attractions. In this case, molecular attraction between A – B is weaker than that of A – A and B – B. Hence the easier it is to break the bonds and boil faster at a lower temperature than that of either of A and B.
5. What deviation is shown by a mixture of equimolar phenol and aniline?
a) Negative deviation
b) Positive deviation
c) No deviation
d) Alternating positive and negative
Explanation: Phenol and aniline both exhibit greater magnitude of hydrogen bonding due to the presence of hydroxyl and amine group, both possessing highly electronegative atoms of oxygen and nitrogen, respectively. The intermolecular hydrogen bonding is far greater than the degree of intramolecular hydrogen bonding which requires more thermal energy to break. Thus, there is an increase in the boiling point of the mixture.
6. If ethanol and chloroform are present in a molar ratio of 2:3 then what is the vapor pressure at 20° C if vapor pressures of pure liquids are 5.95 kPa and 21.17 kPa, respectively?
a) 16.692 kPa
b) 15.082 kPa
c) 8.731 kPa
d) 12.038 kPa
P0eth = 5.95 kPa
P0chl = 21.17 kPa
Mole ratio of ethanol ∶ chloroform = 2 ∶ 3
Total number of parts = 2 + 3 = 5
Therefore, mole fraction of ethanol, Xeth = 2/5 = 0.4
Mole fraction of chloroform, Xchl = 3/5 = 0.6
From Raoult’s law, pA = p0A x XA
Peth = 5.95 x 0.4 = 2.38 kPa
Pchl = 21.17 x 0.6 = 12.702 kPa
From Dalton’s law, Ptotal = Peth + Pchl
Ptotal = 2.38 + 12.702 = 15.082 kPa.
7. Considering a binary solution of components A and B obeys Raoult’s law, which of the following is true?
a) Total vapor pressure cannot be related to mole fraction of only one component
b) Total vapor pressure of one component varies non-linearly with another component
c) A plot of vapor pressures of both components gives a linear plot
d) Total vapor pressure of solution always decreases with increase in mole fraction of a component
Explanation: Raoult’s law states that the partial pressure of each component in the solution varies directly with its mole fraction in the solution. It is formulated as ptotal = pA + (pB – pA)xB. From this, it is seen that when a graph is plotted, it gives a linear plot passing through origin. Total vapor pressure can be expressed as mole fraction of one component and that it varies linearly with the latter. However, total vapor solution may decrease or increase with increase in mole fraction of a component.
8. A volatile liquid with vapor pressure 85 kPa (at sea level, 25° C) is taken to the peak of Mt. Everest. Which of the following is true?
a) The vapor pressure of the solution decreases
b) The solution will condense quickly than when at sea level
c) The solution will vaporize quickly than when at sea level
d) The vapor pressure of the solution increases
Explanation: The vapor pressure of a liquid is independent of the pressure of surroundings. Any solution boils when its vapor pressure equals the surrounding atmospheric pressure. The atmospheric pressure decreases with increase in altitude. Due to this, the vapor pressure equals the atmospheric pressure in a shorter period of time when compared to at the solution being present at sea level. Therefore, the solution will vaporize quickly as the bowling point will reached faster.
9. Which of the following is caused by the addition of a non-volatile solute to a solvent?
a) Reduction in equilibrium of vapor pressure of solution
b) Increase in melting point of the solution
c) Decrease in the boiling point of the solution
d) Osmosis of solute in the solution
Explanation: This phenomenon is known as ‘relative lowering of vapor pressure’ which is a very common colligative property. When a non-volatile solute is added to a solvent, the upper surface of the solvent is covered partially. Hence, the solvent molecules do not bear maximum freedom to escape into the space as vapors. Consequently, there is a decrease in vapor pressure when compared towhat it would have been with pure solvent and no solute at all.
10. What does the vapor pressure of solvent containing a non-volatile solute, in a closed system directly vary with?
a) Mole fraction of solute
b) Mole fraction of solvent
c) Molarity of solute
d) Molarity of solvent
Explanation: More the solute, lower is the vapor pressure of the solvent when compared to that when it is in its purest form. In case of greater amount of non-volatile solute, more solvent can be added to raise the vapor pressure. Therefore, it is always directly proportional to the mole fraction of the solvent.
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