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