Which unit is used to express relative masses?

• A. Kilograms
• B. Atomic mass units
• C. Grams per mole
• D. Joules

Answer: (B)

The idea being tested is how we express masses of atoms relative to a standard. For this, the atomic mass unit (amu, also called the unified atomic mass unit) is used. It’s defined so that one atom of carbon-12 has exactly 12 amu, which makes it convenient to compare the masses of different atoms on the same scale. When we say the relative mass of an element, we’re usually referring to its atomic mass expressed in amu (often written as Ar, the relative atomic mass). This is different from kilogr ams or grams per mole, which describe actual masses or molar masses, and from joules, which measure energy.

So, the unit that expresses relative masses of atoms is the atomic mass unit.