To find the missing mass when you are given the mass and atomic weight, you can use the formula:
Missing mass = Atomic weight - Given mass
The atomic weight represents the weighted average of the masses of all the naturally occurring isotopes of an element, while the given mass refers to the specific mass value you have.
By subtracting the given mass from the atomic weight, you can determine the difference, which represents the missing mass.
Please note that both the atomic weight and the given mass should be expressed in the same unit for accurate calculations, typically in atomic mass units (u) or unified atomic mass units (u).