0.7998g sample of MCl₃ had 40.0mL of 0.300M Na₂S added to it:

2MCl3 3Na₂S -> M₂S₃ +6NaCl

Enough water was then added to make 100.0mL of solution. A 15.0mL of aliquot of the solution ws taken and reacted with 18.0mL of 0.500M AgNO₃, just enough to react with all of the excess Na₂S in the aliquot:

Na₂S + 2AgNO₃-> 2NaNO₃ +Ag₂S

What was the metal, M?