Does equal rest mass always equal equal energy?

[Adam Rifai]

Since C is constant, so all matter of equal rest mass have the same E ?!
e.g. an equal rest mass of water and rest mass of Uranium have the same energy ?

 

[Orodruin]

Yes, the mass energy related to 1 kg of water is the same as the mass energy in 1 kg of uranium.

 

[PeterDonis]

all matter of equal rest mass have the same E ?

Assuming the matter is at rest, yes. For matter in motion, $E = mc^2$ is not correct; the correct formula is $E^2 = m^2 c^4 + p^2 c^2$, where $p$ is the momentum. So pieces of matter with the same $m$ but different $p$ (because they are moving at different speeds) will have different $E$.