In order to determine the number of moles that are in #24.65"g"# of aluminum chloride #("AlCl"_3")#, you must know the mass of one mole of #"AlCl"_3"#, which is called its molar mass. You can do that by multiplying the subscript for each element times its atomic weight in grams/mol (from the periodic table), and adding them.
Molar Mass #"AlCl"_3"#
#(1xx26.982"g/mol Al")+(3xx35.45"g/mol Cl")=133.342"g/mol"#
Number of Moles in #24.65"g AlCl"_3"#
Divide the given mass of aluminum chloride by its molar mass.
#(24.65color(red)cancel(color(black)("g")) "AlCl"_3)/(133.342color(red)cancel(color(black)("g"))/"mol") ="0.1849 mol AgCl"_3#