Einstein’s law of the equivalence of mass and energy :
According to it, the mass and energy are inter convertible aspects of the same property, and when a body gives up energy, it also loses mass. Numerically this law has been stated as E = mc2 (i.e.m. = E/C2), where E is energy, m is the mass and c is the velocity of light in vacuum. This law holds good in all cases but in ordinary chemical or molecular processes the change in mass is so small that it is not possible to measure or observe it.