Question

0.702g arsenic combines with 0.225g of oxygen to make an oxide of arsenic. When heated this produces a gas with a volume of 52cm3.

How do i work out the molecular formula of the oxide when its in a gaseous state?

The weight ratio will tell you, after dividing by atomic weights of AS and O, that the ratio of As to O is 2 to 3. That means the formula for the oxide is
As2O3 or some multiple.

Knowing the volume of the product does not help unless you also know the pressure and temperature. If it is STP (273 K and 1 atm), you would have .052 liter/22.4 liters/mole = 2.32*10^-3 moles weighing 0.702 + 0.225 = 0.927 g. That is 400 g/mole. That would tend to imply a vapor formula of As4O6.

Thnk you so much