how do i find the mass number of chemicals

I would think that you would add up the components masses (molecular masses [from periodic table) that make up the chemical...
like...HCl (H= 1.008g/mol, Cl= 35.45g/mol) added= 36.458g/mol

hope this helps=)

Elements have a mass number. Compounds have molar masses but not mass numbers in the strict sense of the term. The mass number of an element is the number of protons + neutrons. For example, oxygen has an atomic number of 8 meaning it has 8 protons in the nucleus If we are discussing ¹⁶O, then 16 is the mass number (and it would have 8 neutrons).