Hint: Before solving this question, we should know that The addition of the atomic masses of components of a compound helps to determine the formula mass of an ionic compound. It is expressed in grams/mole. It is known as Gram formula mass.
Complete answer: The summation of atomic weights of the atoms in the empirical formula is known as formula mass. It is only applicable in the case of ionic compounds. Some examples are- Sodium chloride (NaCl) is an ionic compound which does not have discrete molecules as its constituent units. The cation and anion are arranged in a three-dimensional structure. The formula is used to calculate the formula mass and not molecular mass in the solid state, NaCl does not exist as a single entity. The amount of the compound which has a mass in grams equals the formula mass in the atomic mass unit is the gram formula mass. Every element consists of an atom that has a characteristic mass. So, every molecule of a compound has a characteristic formula mass.
Note: Molecular mass and formula mass tells us the same thing. Formula unit mass helps to tell the mass of ionic compounds whereas molecular mass helps us to tell the mass of covalent compounds. The reason we use them differently is that molecules are covalent compounds which means they only have non–metals. The ionic compounds are not molecules, so we don’t find the molecular mass for an ionic compound, so we name it the formula mass in the case of ionic compounds.