Crystals (empirical formula)?

Question

A crystal packs with a titanium ion on each corner of the cell, an oxide ion on each edge and a calcium ion in the center of the cell. What is the empirical formula of this mineral?

A. CaTiO

B. CaTiO2

C. CaTiO4

D. CaTiO3

E. CaTi2O3

i can never understand crystals, please explain, i believe the answer was b

Answer 1

In one unit cell, there are 8 Ti, 1 Ca, and 12 O, but the Ti and O are shared with neighboring cells.  Each Ti atom (corner) is shared between 8 unit cells, so there is one per unit cell.  Each O (edge) is shared between 4 cells, so there are 12/4=3 O per cell.  Thus, the empirical formula should be D: CaTiO3.

This formula also makes sense from the oxidation states that Ca, O, and Ti commonly assume.  Ca is always 2+, O is almost always 2-, and Ti is usually 4+ (Ti2+, required for choice B, exists, but is unstable).  CaTiO3 is the mineral perovskite, while CaTiO2 does not occur naturally.

Answer 2

you need the mass of the compound divided by the % compoition of each of the 3 elements involved (Ca, Ti, O).  Then divide each of those #'s by the lowest number.  lets say you get Ca=1, Ti=1, but O=2.  

That would mean ur emp. formula is CaTiO2