What will be the mass of anhydrous alum left over if one starts with 100.00 g of hydrated alum?

The formula for hydrated alum is KAl(SO₄)₂.12H₂O and for anhydrous alum the water molecules are lost. First, we calculate the percentage of water in hydrated alum so that we know how much mass will be lost:
(12 x 18) x 100/ (474  + 12 x 18)
= 31.3%
Therefore, 31.3 grams of water will be lost form the 100 gram sample of hydrated alum and the remaining mass will be 100 - 31.3 = 69.7 grams