All Free Chemistry MCQs with Answers

61. The molar mass of carbon dioxide, CO2, is

• A. 44 g per mole
• B. 28 g per mole
• C. 12 g per mole
• D. 32 g per mole

Explanation: Adding one carbon at 12 and two oxygens at 16 each gives 12 + 32, that is 44 g per mole. The value 28 belongs to carbon monoxide and 32 to oxygen gas, both of which appear here because they are the neighbouring molecules students confuse it with. Molar mass in grams per mole is numerically equal to relative molecular mass.

62. The relative atomic mass of an element is

• A. the mass in grams of one atom of the element
• B. the average mass of its atoms compared with one twelfth the mass of a carbon 12 atom
• C. the number of protons in its nucleus
• D. always a whole number

Explanation: Relative atomic mass is a ratio, so it has no units, and it is an average weighted by the natural abundance of the isotopes. That averaging is why chlorine is quoted as 35.5 rather than as a whole number, since it is a mixture of chlorine 35 and chlorine 37. The number of protons is the atomic number, an entirely different quantity.

63. The molarity of a solution is defined as the number of moles of solute dissolved in

• A. 1000 g of solvent
• B. one litre of solvent
• C. one cubic decimetre of solution
• D. one mole of solvent

Explanation: Molarity refers to the total volume of the final solution, not to the volume of solvent used, which is why a standard solution is made up to the mark in a volumetric flask rather than by adding a measured volume of water. Moles per 1000 g of solvent defines molality instead, a quantity that does not change with temperature. The distinction matters whenever a solution is heated.

64. Which of the following samples contains the greatest number of atoms?

• A. 1 mole of helium gas
• B. 1 mole of oxygen gas, O2
• C. 1 mole of water, H2O
• D. 1 mole of ammonia, NH3

Explanation: Every sample contains one mole of molecules, so the answer depends on the number of atoms in each molecule: helium has one, oxygen two, water three and ammonia four. One mole of ammonia therefore contains 4 x 6.02 x 10^23 atoms, more than any of the others. Confusing moles of molecules with moles of atoms is exactly the error this question is designed to expose.

65. A liquid takes the shape of its container but has a fixed volume because its molecules

• A. are held in fixed positions in a lattice
• B. are close together but free to slide past one another
• C. are very far apart and move independently
• D. have no kinetic energy at all

Explanation: Intermolecular forces in a liquid are strong enough to keep the molecules in contact, which fixes the volume, but not strong enough to hold them in place, so they flow and take the shape of the vessel. Fixed positions in a lattice describe a solid, and wide separation with independent movement describes a gas. A liquid is therefore intermediate in both structure and freedom of movement.

66. Surface tension in a liquid arises because

• A. molecules at the surface are pulled inwards and sideways only, with no upward pull
• B. the surface molecules move faster than those inside
• C. air presses down on the surface
• D. the liquid evaporates from the surface

Explanation: A molecule inside the liquid is pulled equally in all directions, but one at the surface has no neighbours above it, so the net inward pull makes the surface behave like a stretched skin and contract to the smallest possible area. This is why free drops are spherical and why a needle can float on water. Surface tension falls as temperature rises, because the molecules are harder to hold together.

67. The viscosity of a liquid

• A. increases as the temperature rises
• B. is independent of temperature
• C. decreases as the temperature rises
• D. is a measure of how quickly it evaporates

Explanation: Viscosity is resistance to flow and it depends on the intermolecular forces, so heating a liquid gives the molecules more kinetic energy to overcome those forces and the liquid flows more freely. This is why engine oil thins in a hot engine and why honey pours easily when warmed. Gases behave in the opposite way, becoming more viscous when heated.

68. Evaporation of a liquid causes cooling because

• A. the surface of the liquid contracts
• B. cold air replaces the escaping vapour
• C. the vapour formed is colder than the liquid
• D. the fastest moving molecules escape, lowering the average kinetic energy of those left behind

Explanation: Only molecules with above average kinetic energy can break free from the surface, so the average energy of the remaining liquid falls and the temperature drops with it. This is why sweating cools the body and why water in an earthen pot stays cool. Evaporation happens at all temperatures, unlike boiling, which occurs only at one specific temperature for a given pressure.

69. The vapour pressure of a liquid is the pressure exerted by its vapour when

• A. the liquid has completely evaporated
• B. the vapour is in dynamic equilibrium with the liquid in a closed container
• C. the liquid is boiling in an open vessel
• D. the container is completely empty of air

Explanation: In a closed vessel evaporation and condensation eventually occur at the same rate, and the constant pressure of vapour at that point is the equilibrium vapour pressure of the liquid at that temperature. It depends only on the liquid and the temperature, never on the volume of the container or the amount of liquid present. In an open vessel equilibrium is never reached because vapour escapes.

70. A liquid boils when its vapour pressure becomes equal to

• A. zero
• B. its surface tension
• C. the external atmospheric pressure
• D. the vapour pressure of water

Explanation: Bubbles of vapour can only form and rise through the bulk of the liquid when the vapour pressure inside them matches the pressure pushing down from outside. Because atmospheric pressure falls with altitude, water boils below 100 degrees Celsius on a mountain, which is why food takes longer to cook there. A pressure cooker uses the same relationship in reverse.