Suppose 358 g aluminum sulfide react with 124 g water.
Al2S3 + H2O -> Al(OH)3 + H2S unbalanced!
What mass of the excess reactant remains?
Al2S3 + 6H2O -> 2Al(OH)3 + 3H2S
Molecular wt of aluminium sulphide = 2*27+3*32 = 150
Molecular wt of water = ...
The answer is given step-by-step equationally.