Gay-Lussac's Law is an ideal gas relationship between the pressure and temperature of a gas held at constant volume. We cannot actually use this relationship in determining the volume of the gas that will be produced upon the decomposition reaction of H2CO3.

H2CO3 is a weak acid that forms an aqueous solution when dissolved in water. It creates an equilibrium in the solution with the equation:

H2CO3 = CO2 + H2O

The best way to determine the volume of the oxygen that will be produced is the use of the ideal gas equation, PV = nRT.

Using stoichiometry, you can determine the amount of oxygen in moles. With the given temperature and pressure, you can solve for the volume of the oxygen that can be produced in the reaction.