What term describes the sum of the relative atomic masses of all atoms in a molecule?

• A. Relative molecular mass
• B. Relative atomic mass
• C. Relative formula mass
• D. Avogadro's constant

Answer: (A)

This question tests how we describe the mass of a molecule on the relative atomic mass scale. The relative atomic mass is the mass of a single atom compared with 1/12 of a carbon-12 atom. When you add up the relative atomic masses of every atom in a molecule, you obtain the relative molecular mass. So for a molecule like water, with two hydrogens and one oxygen, the calculation is 2×1 + 16 = 18, giving a relative molecular mass of 18. This is the term that describes the total mass of all atoms in a molecule. The term relative formula mass is used for formula units of ionic compounds, not discrete molecules, and Avogadro's constant is a number describing how many particles are in a mole, not a mass.