To determine the formula of the oxide, let's analyze the given information:

  Stoichiometrically, 2 X +  3/2 O\(_2\) →  Oxide

Given :

1. 1 litre of monoatomic gas X combines with 1.5 litres of an oxide.

2. The reaction is a combination reaction, where X reacts with oxygen to form an oxide.

If the atom of the elements are X and  O\(_2\)

Reacted volumes were given as 1dm\(^3\)  1.5dm\(^3\)

Stoichiometrically, moles of molecules 2 moles  3 moles

X(monoatomic) and oxygen(diatomic)  2 x 1 3 x 2 = 6 (3 moles of O\(_2\) = 6 oxygen atoms)

Divide ratios by 2  \(\frac{2}{2}\)  \(\frac{6}{2}\)

The formula ratio : 1 : 3

Therefore, the formula of the oxide is XO\(_3\)