All Free Chemistry MCQs with Answers

71. Which of the following liquids would be expected to have the highest vapour pressure at room temperature?

• A. Water
• B. Glycerol
• C. Ether
• D. Mercury

Explanation: Ether has weak dispersion forces between its molecules, so a large fraction escape the surface easily and it is highly volatile, which is why it evaporates almost as soon as it is spilt. Water and glycerol are held by extensive hydrogen bonding and mercury by metallic bonding, all of which are far stronger. Vapour pressure and boiling point always run in opposite directions.

72. A hydrogen bond forms when hydrogen is covalently bonded to

• A. carbon, silicon or sulphur
• B. any halogen
• C. a metal
• D. a small, highly electronegative atom such as fluorine, oxygen or nitrogen

Explanation: The electronegative atom pulls electron density away, leaving the hydrogen nucleus unusually exposed so that it attracts a lone pair on a neighbouring molecule. Only fluorine, oxygen and nitrogen are small and electronegative enough for the effect to be strong. Carbon is not electronegative enough, which is why hydrocarbons show no hydrogen bonding and are gases or volatile liquids.

73. The boiling point of water is far higher than that of hydrogen sulphide, although sulphur lies below oxygen in the same group, because

• A. water molecules are hydrogen bonded to one another
• B. water has a larger molar mass
• C. hydrogen sulphide is an ionic compound
• D. water molecules are non polar

Explanation: Extensive hydrogen bonding between water molecules must be overcome before they can escape, so a great deal of energy is needed and water is liquid at room temperature. Hydrogen sulphide has a larger molar mass yet boils at minus 60 degrees Celsius, which proves that molar mass is not the deciding factor. Sulphur is too large and not electronegative enough to support hydrogen bonding.

74. Hydrogen bonding is responsible for all of the following EXCEPT

• A. the high boiling point of ammonia compared with phosphine
• B. the solubility of ethanol in water
• C. the low density of ice compared with water
• D. the electrical conductivity of molten sodium chloride

Explanation: Molten sodium chloride conducts because it is an ionic compound whose ions become mobile when the lattice melts, and there is no hydrogen in the compound at all. The other three are classic consequences of hydrogen bonding: raised boiling points, mutual solubility with water and the open lattice of ice. Recognising when a substance simply cannot hydrogen bond is the quickest route to the answer.

75. Ice is less dense than liquid water because in ice each water molecule

• A. loses one of its hydrogen atoms
• B. is hydrogen bonded to four others in an open tetrahedral arrangement
• C. moves faster than in the liquid
• D. is compressed closer to its neighbours

Explanation: Every molecule uses its two hydrogens and its two lone pairs, so a rigid open cage with large empty spaces is formed, and this occupies more volume than the same molecules jostling in the liquid. Water is therefore anomalous in expanding on freezing, which is why pipes burst in winter and why ice floats. Almost every other substance contracts as it solidifies.

76. Water has its maximum density at

• A. 0 degrees Celsius
• B. 100 degrees Celsius
• C. 4 degrees Celsius
• D. minus 4 degrees Celsius

Explanation: Cooling water from room temperature increases density as normal, but below 4 degrees the open hydrogen bonded structure begins to form and the density falls again, so the maximum occurs at 4 degrees. This is why the bottom of a deep lake stays at about 4 degrees while ice forms at the surface, allowing aquatic life to survive the winter. At 0 degrees water is already less dense than at 4.

77. Water has an unusually high specific heat capacity because

• A. it is a very light molecule
• B. it is a good electrical conductor
• C. a large amount of energy is used breaking hydrogen bonds before the temperature rises
• D. it has a low boiling point

Explanation: Much of the heat supplied goes into breaking hydrogen bonds rather than into increasing the kinetic energy of the molecules, so the temperature rise per joule added is small. This makes water an excellent coolant in car radiators and gives coastal areas milder climates than inland regions. Its high latent heat of vaporisation has the same underlying cause.

78. Capillary rise of water in a narrow glass tube occurs because

• A. the adhesive forces between water and glass are stronger than the cohesive forces within water
• B. the cohesive forces are stronger than the adhesive forces
• C. the air pressure inside the tube is greater
• D. water is denser than glass

Explanation: Water wets glass because it is attracted to the silica surface more strongly than to itself, so it climbs the walls and drags a column up, forming a concave meniscus. Mercury shows the opposite behaviour, with stronger cohesion, so it is depressed in the tube and forms a convex meniscus. The narrower the tube, the higher the rise.

79. The energy required to convert one mole of a liquid into vapour at its boiling point is called the

• A. molar heat of fusion
• B. molar heat of vaporisation
• C. lattice energy
• D. activation energy

Explanation: Molar heat of vaporisation measures the energy needed to overcome the intermolecular forces completely, so it is large for hydrogen bonded liquids such as water at about 40.7 kJ per mole. Heat of fusion is the smaller quantity needed to melt a solid, because melting loosens the lattice without separating the particles. Neither is related to lattice energy, which concerns ionic solids.

80. As the temperature of a liquid rises, its vapour pressure

• A. decreases
• B. remains constant
• C. increases
• D. first increases and then decreases

Explanation: Higher temperature means a larger fraction of molecules have enough kinetic energy to escape the surface, so more vapour is present at equilibrium and the pressure rises, following an exponential rather than a straight line relationship. The boiling point is simply the temperature at which this rising curve meets the external pressure. This is why a liquid in a sealed container can burst it when heated.