The most abundant metal in Earth's crust is aluminum, found mostly in the form of clays. There are no economical routes for extracting aluminum from clay. However, bauxite ore, impure hydrated aluminum oxide, is found in hot humid regions, such as Australia, Guinea, and Brazil, and it can be purified and refined to make the metal. After the first purification step, hydrated aluminum oxide (Al₂O₃ . xH₂O) is obtained. When 100.0 g of this solid was heated, and the water driven off, 65.36 g of Al₂O₃ remained. How many water molecules (x in the molecular formula) were there in the hydrate?

Question

Quizplus · Accepted Answer

The mass of water driven off is 100.0 g - 65.36 g = 34.64 g. The molar mass of Al2O3 is 101.96 g/mol, and the molar mass of H2O is 18.02 g/mol. The moles of Al2O3 are 65.36 g / 101.96 g/mol = 0.641 moles. The moles of H2O are 34.64 g / 18.02 g/mol = 1.923 moles. The ratio of moles of H2O to Al2O3 is approximately 3:1, indicating there are 3 water molecules per formula unit of Al2O3.

The Most Abundant Metal in Earth's Crust Is Aluminum, Found