well 12.15g is the mass I weighed for my product here's how I got 3.2g theoretical
1.115g starting aluminum foil x 2/1 (mole ratio) xmol/342.2g = 0.006755 mols x 474.4g/mol = 3.2g of KAI(SO4)2*12H20
Equation I used is
Al2(SO4)3 + K2SO4 + 24H20 --> 2KAI(SO4)2*12H2O