The formula of aluminium sulphide is al₂s₃. explain why the formula has a ratio of two aluminium ions for every three sulphide ions.

Molar mass of Al = 27.0 g/mol
Molar mass of S = 32.1 g/mol
Molar mass of Aluminium sulfide = 27.0 x 2 + 32.1 x 3 = 150.3 g/mol
% of Al (there are 2 Al atoms) in Aluminium sulfide = 
% of S (there are 3 S atoms) in Aluminium sulfide = 
Assume the percentages to be masses, so:
In 100.0 g of Aluminium sulfide, n (Al) = 
In 100.0 g of Aluminium sulfide, n (S) = 
Convert values to a molar ratio,
1.33 : 2.00
Divide by smallest ratio value,
1 : 1.50
Multiply each ratio value by a factor of 2 to eliminate the decimal,
2 : 3

Hence why the empirical formula of Aluminium sulfide is 

The think of process to make something successful

Molarity is moles of solute over liters of solution. idk if that is what you are talking about, but you can convert moles of solute to grams of solute using dimensional analyzes by using molar mass found on the periodic table.