Question

Spinel is a mineral that contains 37.9% Al, 17.1% Mg, and 45.0% O by mass, and has a density of 3.57g cm-3 . The unit cell is cubic with an edge length of 809 pm (This is a lot bigger than question #2 and #4 for a reason. Think about this after you have calculated an answer). How many atoms of each type are in the unit cell?

Answer 1

The edge length of unit cell

The volume of unit cell

Density of unit cell is 3.57 g/cm3

Mass of unit cell

The percent composition is 37.9 % Al, 17.1 % Mg and 45.0 % O.

Mass of Al in the unit cell

Mass of Mg in the unit cell

Mass of O in the unit cell

The atomic masses of Al, Mg and O are 27 g/mol, 243 g/mol and 16 g/mol respectiely.

Number of Al atoms in unit cell

Number of Mg atoms in unit cell

Number of O atoms in unit cell

This is a lot bigger than question #2 and #4. This is because one unit cell of edge length 809 pm contains several small unit cells having smaller edge length.