How would you determine the formula and molar mass of aluminum sulfide?

##"Aluminum sulfide"## ##-=## ##Al_2S_3##.

So how did I know?

Aluminum commonly forms an ##Al^(3+)## ion.

And sulfur (like congeneric oxygen) commonly forms a ##S^(2-)## ion.

To make a neutral salt, the charge must balance hence, ##Al_2S_3##.

Note that these ionic identities can be predicted by the position of these on . How?

And as to the masses, sulfur has a mass of ##32.06*g*mol^-1##, and aluminum has a mass of ##26.98*g*mol^-1##.

And thus the molar mass of ##"aluminum sulfide "= (2xx26.98+3xx32.06)*g*mol^-1=??*g*mol^-1##.