The mass number of a nucleus (except hydrogen) is:

The mass number of a nucleus (except Hydrogen) is: number of protons + number of neutrons.

A=Z+N

A=mass number=protons + neutrons.
Z=atomic number=number of protons.
N=number of neutrons. 

In the case of Hydrogen it depends of isotope of Hydrogen . 
the hydrogen has three isotopes, 
protium : A=Z, because N=0
deuterium:  A=Z+N;   N=1
 tritium: A=Z+N;    N=2 

You will hear a hissing sound because of the carbon dioxide escaping the can or bottle