How do i find the volume of a gas at stp

you would have to divide the mass by the molecular weight of the gas molecules to get the number of moles. Then multiply this by 22.4 Liters / mole to get the volume. For example, if you have 96 g of O2, then you can divide by the molecular weight of O2, which is 32 g/mol, to get 3 moles.

the potential energy of the reactants is 200j.

from the energy diagram, the energy of the product formed is 350j; this means that, this reaction is an endothermic reaction, because it absorbs energy from its environment.