what volume does the "v" in pv denotes? say that in a system where pressure is constant Mg reacts with O2.when dealing with above reaction thermodynamically, HI=UI+PVI where H is the initial enthalpy of the system UI is the initial internal energy of the system. And my question is that what does the VI denotes in the equation.Is it the volume of magnesium and oxygen? Likewise when we write the equation after the reaction, what does the VF denotes?Is it the volume of magnesium oxide formed ? can anyone explain?