Crystallization

In an FCC lattice, there are 4 atoms per unit cell. The number of octahedral voids equals the number of atoms (4), and the number of tetrahedral voids is twice the number of atoms (8), so they are not equal (ruling out option 1). Tetrahedral voids are located along the body diagonals, not at the edge centers, which are occupied by atoms (ruling out option 2). Octahedral voids are indeed located at the body center (1/2, 1/2, 1/2) and edge centers (e.g., 1/2, 0, 0), making option 3 correct. Finally, the packing efficiency of FCC and HCP lattices is the same (74%), so option 4 is incorrect. Thus, the correct answer is option 3.

For an FCC lattice, the number of atoms per unit cell ( ) is 4. The density ( ) of a crystal is given by the formula: where is the atomic weight, is the edge length, and is Avogadro's number. Rearranging for : Given: , , , and . First, calculate : Now, substitute the values: Comparing with the options, the closest value is 63.178 u. Thus, the correct answer is 63.178 u.

The water of crystallization in salts such as borax, kernite, and bauxite is well-known. Borax has 10 water molecules, kernite has 4, and bauxite has 2. Thus, the correct answer is .