Question

The formula unit mass of a substance is the:

• A. sum of the atomic masses of all atoms in a formula unit of a compound
• B. relative atomic masses of atoms
• C. double the numerical value of the atomic mass
• D. None of above

Answer 1

Answer: (A)

sum of the atomic masses of all atoms in a formula unit of a compound

The formula mass of a substance is defined as the sum of the atomic masses of constituent atoms in a compound. This is generally used for compounds which do not contain discrete molecules, but ions as their constituent units. Example: NaCl (sodium chloride) made of sodium and chlorine ions. Formula unit mass of NaCl = (1 $\times$ 23) + (1 $\times$ 35.5) = 58.5 u