Answer
12
Work Step by Step
Number of moles of water:
$2.16\ g\div 18.015\ g/mol=0.1199\ mol$
Number of moles of potassium to aluminum sulfate:
$(4.74-2.16)\ g\div 258.20\ g/mol=0.00999\ mol$
Ratio:
$0.1199/0.00999=12.0$
12 molecules of water per formula unit.
