Can we say that the coulomb volt/c^2 is an equivalence to mass? I am asking because I just don't know.

No. However if you take two charges and place them next to each other then the mass of the system of two charges is different than the sum of the two masses. That's due to the fact that there's a change in potential energy of the two particle system. Whether there's an increase or decrease depends on whether the charges are positive or negative. If the charges are restrained so as to be held at a fixed distance from each other then it gets more complicated. For example; if you nail one charge to one end of a stick and another charge at the other end of stick then there will be stress in the stick. It's a fact that stress has inertia so you have to take that into account to get the right value of the mass of the system.

There was a very interesting article in

**The American Journal of Physics** about 10 years ago on this subject. The article is

**The inertia of stress** by Rodrigo Medina,

*Am. J. Phys. *,

**74**, 1031 (2006). You can download it from:

http://arxiv.org/abs/physics/0609144Very cool stuff and very important. This is not a well known fact in physics. A lot of special relativity authors either omit it or are not familiar with it.