In a molecule hydrogen and oxygen are present in the ratio 1: 8 by mass. Deduce the molecular formula of the compound. (Atomic mass: H=1.0u,O=16.0u).

Dear Student,


First let us derive the no. of moles of hydrogen and oxygen present in the compound, using , n =m/M,
where n is the no. of moles, m is the mass in given unit and M is the molar mass.

Then no. of moles of Hydrogen, n_H = 1/1 = 1
no. of moles of Oxygen, n_O = 8/16 = 0.5.

Then the mole ratio is obtained by n_O/n_H = 0.5 moles of O/ 1 mole of H = 1 mole of O/2 moles of H.
i.e., In the compound for every mole of oxygen there are 2 moles of Hydrogen.
Then the empirical formula for the compound can be obtained as H₂O.
Then the molecular weight = 2$\times$1 + 1$\times$16 = 18 units
Molecular formula is a multiple of the empirical formula.

The given molecule is also of 18 units of mass. (it is given that  that the masses are in ratio 1:8, and there is no specific value for mass of the molecule given so we assume the simplest possible molecular mass i.e, 18 grams).

Hence the molecular formula will also be H₂O.

(here the actual units of mass is grams, but is not used to avoid any confusion)


Regards