Question

An oxide of niobium has a cubic unit cell in which there are oxide ions at the middle of each edge and niobum atoms at the center of each face. what is the empirical formula of this oxide?

Answer 1

Question 1:

1. One form of niobium oxide crystallizes in the unit cell shown above.

A. How many niobium atoms per unit cell? 3

B. How many oxygen atoms per unit cell? 3

C. What is the formula of niobium oxide? Nb2O5

Answer: A) 3 B) 3 C) Nb205

Question 2:

2. Quartz (left) and glass (right) are both forms of silicon dioxide. A piece of quartz breaks into a collection of smaller regular crystals with smooth faces. A piece of glass breaks into irregular shards. Use molecular structures to explain why the two solids break so differently. Select all that apply.

1. the bonding and geometry in quartz shows regular repeating patterns *

2.  the bonding and geometry in quartz shows irregular repeating patterns

3. the bonding and geometry in glass shows irregular repeating patterns *

4. the atomic geometries of the two solids can explain why they break so differently  *

5. the atomic geometries of the two solids cannot explain why they break so differently

6. the bonding and geometry in glass shows regular repeating patterns

Answer: 1,3,4

3. Solid silver adopts a face-centered cubic lattice. The metallic radius of a silver atom is 144 pm.

a. How many silver atoms occupy one unit cell of solid silver? 4

b. Calculate the length (in pm) of one side of the unit cell. 407

c. Calculate the volume (in m3) of one the unit cell.  6.74 x10^-29

d. What percentage (expressed to four significant figures) of the volume is empty? 25.95

Answer: a)4 b)407  c)6.74 x10^-29   d)25.95

4. Why hexane is insoluble in water? Select all that apply.

1. Water is polar solvent. *

2. Polar solvents dissolve both polar and non-polar compounds.

3. Polar solvents dissolve only polar compounds. *

4. Solubility does not depends on the polarity of the solvents and reagents.

5. Hexane is non-polar compound. *

Answer: 1,3,5

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Answer 2

Answer:

Empirical formula: $NbO_2$

Explanation:

• One oxide ion in the middle of each edge => cubes have 12 edges => O=12
• One niobium atom at the center of each face => cubes have 6 faces => Nb=6

Cell formula: $Nb_6O_{12}$

Empirical formula: $NbO_2$

This formula corresponds to a niobium valence of +4