What is the formula for zinc chloride with a formula weight of 138.28 amu?

The molecular formula for zinc chloride is ##ZnCl_2##.

The most common formal oxidation state of zinc is +2, and the most common oxidation state of chlorine is -1. In order to form a neutral molecule, two chlorine atoms are required for each zinc atom.

The molar mass of zinc chloride is:

1 ##Zn## atom - 65.409 g/mol 2 ##Cl## atoms - ##2 times 35.453## = 70.906 g/mol Total = 136.315 g/mol

(close to the value quoted in the question)