This set of Engineering Chemistry Multiple Choice Questions & Answers (MCQs) focuses on “Boiling Point”.
1. On increasing the temperature of a liquid, its vapour pressure ____________
a) Decreases
b) Remain constant
c) Increases
d) None of the mentioned
View Answer
Explanation: The vapour pressure of a liquid increases with an increase in temperature. Vapour pressure or equilibrium vapor pressure is defined as the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases at a given temperature in a closed system.
2. The boiling point of a liquid is the temperature at which the vapour pressure ____________
a) Is equal to the internal pressure
b) Is equal to the external pressure
c) Is greater than an internal pressure
d) Is lesser than an internal pressure
View Answer
Explanation: The boiling point of a liquid is the temperature at which the vapour pressure is equal to external pressure. The boiling point of a liquid varies depending upon the surrounding environmental pressure.
3. The normal boiling point of a liquid is the temperature at which it boils when the external pressure is ____________
a) 1 atm
b) 2 atm
c) 3 atm
d) 5 atm
View Answer
Explanation: The normal boiling point of a liquid is the temperature at which it boils when the external pressure is 1 atm. The boiling point of a liquid varies depending upon the surrounding environmental pressure.
4. If the non-volatile impurities contaminate the liquid, its boiling point gets ____________
a) Depressed
b) Elevated
c) Remains same
d) None of the mentioned
View Answer
Explanation: If the non-volatile impurities contaminate the liquid, its boiling point gets elevated. This is one of the most important colligative property. Example: salt water boils at a higher temperature than pure water.
5. The energy required to vaporize 1 mole of a liquid is known as ____________
a) Molar heat of fusion
b) Molar heat of freezing
c) Molar heat of boiling
d) Molar heat of vaporisation
View Answer
Explanation: The energy required to vaporize 1 mole of a liquid is known as the molar heat of vaporisation. It is the amount of heat necessary to boil (or condense) one mole of a substance at its boiling point.
6. At 1 atm pressure, the boiling point of water is ____________
a) 0 0C
b) 1000C
c) -1000C
d) None of the mentioned
View Answer
Explanation: The boiling point of water at 1 atm pressure is 1000C. The boiling point of a liquid varies depending upon the surrounding environmental pressure. A liquid in a partial vacuum has a lower boiling point than when that liquid is at atmospheric pressure.
7. The boiling point and molar heat of vaporisation is dependent on ____________
a) Strength of the inter-molecular forces
b) Composition of the liquid compound
c) Size of the molecules
d) All of the mentioned
View Answer
Explanation: The boiling point and molar heat of vaporisation is dependent on the strength of the inter-molecular forces. The boiling point of a liquid varies depending upon the surrounding environmental pressure.
8. Select the incorrect statement from the following options.
a) Methane and argon have weak dispersion forces so their boiling point is low
b) Diethyl ether has a dipole moment, and the dipole-dipole forces accounts for its moderately high boiling point
c) Water and ethanol have strong hydrogen bonding and hence they possess high boiling point
d) Mercury has weak metallic bonding which is responsible for its low boiling point
View Answer
Explanation: Mercury has strong metallic bonding which is responsible for its high boiling point. All the other options are correct. Methane and argon have weak dispersion forces so their boiling point is low. Diethyl ether has a dipole moment, and the dipole-dipole forces accounts for its moderately high boiling point. Water and ethanol have strong hydrogen bonding and hence they possess a high boiling point.
9. For a given substance, choose the correct statement.
a) ΔHfus is greater than ΔHvap
b) ΔHfus is equal to ΔHvap
c) ΔHfus is smaller than ΔHvap
d) None of the mentioned
View Answer
Explanation: For a given substance, ΔHfus is smaller than ΔHvap.
10. The molar heat of vaporisation (ΔHvap) for water is ____________
a) 40.79 kJ/mol
b) 31.0 kJ/mol
c) 59.0 kJ/mol
d) 9.2 kJ/mol
View Answer
Explanation: The molar heat of vaporisation (ΔHvap) for water is 40.79 kJ/mol. Vapor pressures and boiling points of substances can be affected by the presence of dissolved impurities (solutes) or other miscible compounds, the degree of effect depending on the concentration of the impurities or other compounds.
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