Free Moles and Avogadro's Number MCQs with Answers

11. 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.

12. 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.

13. 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.

14. Consider the equation H2 + O2 gives H2O. What volume of hydrogen gas is required to produce 1 mol of water at standard temperature and pressure?

• A. 11.2 dm3
• B. 22.4 dm3
• C. 10 dm3
• D. 58 dm3

Explanation: Balanced properly the equation is 2H2 plus O2 giving 2H2O, so one mole of water needs one mole of hydrogen, and one mole of any gas occupies 22.4 dm3 at STP. The figure 11.2 dm3 is half a mole and is the trap for anyone who halves the ratio the wrong way. Molar volume is independent of the identity of the gas, which is Avogadro's law.