How do we calculate molecular mass and formula unit mass.

Asked by sabu_sreeja | 28th Oct, 2015, 04:20: PM

Expert Answer:

Both molecular mass and formula mass signify the same thing.

‘Molecular mass’ is used for mass of covalent compounds (molecules). Similarly, ‘Formula unit mass’ is the term used for mass of ionic compounds.

The only reason we use two different terms is because molecules are covalent compounds meaning they have only non-metals. So ionic compounds cannot be a molecule and therefore it would not make sense to find the molecular mass for an ionic compound, so we just call it the "formula mass", when we are talking about ionic compounds.

The molecular mass is the mass of one molecule of a substance. It is calculated by summing up the atomic masses of all the elements present in that particular molecule.


(a) The molecular mass of H2O can be calculated as follows:

H2O = 2 X 1 + 16 = 18 u

(b) The molecular mass of CO2 can be calculated as follows:

CO2 = 12 + 16 X 2 = 44 u

(c) The molecular mass of NH3 can be calculated as follows:

NH3 = 14 + 3 X 1 = 17 u

The formula mass of an ionic compond like sodium chloride will be,

NaCl = 23 + 35.5 = 58.5 u


Answered by Hanisha Vyas | 28th Oct, 2015, 06:54: PM