Which term is the sum of the relative atomic masses for an ionic compound?

Which term is the sum of the relative atomic masses for an ionic compound?

• A. Relative formula mass
• B. Relative molecular mass
• C. Relative atomic mass
• D. Molar mass

Answer: (A)

The concept being tested is how we quantify the mass of an ionic compound in terms of its formula unit. For ionic compounds there isn’t a discrete molecule; we look at the simplest repeating unit, the formula unit, and add up the relative atomic masses of all the atoms in that unit. This sum is called the relative formula mass (often abbreviated Mr). For example, in sodium chloride the formula unit is NaCl. Adding the relative atomic masses gives 23 (Na) + 35.5 (Cl) = 58.5, so the relative formula mass is 58.5. This value also corresponds to the molar mass in g/mol, but the standard term for the sum of the atomic masses in a formula unit of an ionic compound is relative formula mass. Relative molecular mass would apply to molecules, not ionic lattices, and relative atomic mass is just the mass of a single atom, not a whole formula unit.

The concept being tested is how we quantify the mass of an ionic compound in terms of its formula unit. For ionic compounds there isn’t a discrete molecule; we look at the simplest repeating unit, the formula unit, and add up the relative atomic masses of all the atoms in that unit. This sum is called the relative formula mass (often abbreviated Mr). For example, in sodium chloride the formula unit is NaCl. Adding the relative atomic masses gives 23 (Na) + 35.5 (Cl) = 58.5, so the relative formula mass is 58.5. This value also corresponds to the molar mass in g/mol, but the standard term for the sum of the atomic masses in a formula unit of an ionic compound is relative formula mass. Relative molecular mass would apply to molecules, not ionic lattices, and relative atomic mass is just the mass of a single atom, not a whole formula unit.