2.5 Practice Problems | Chemistry

2.5 Practice Problems

Attempt these problems as if they were real exam questions in an exam environment.

Only look up information if you get severely stuck. Never look at the solution until you have exhausted all efforts to solve the problem.

What is the empirical formula of the unit cell for a binary ionic compound shown on the cover of this exam? (A = black particles, X = gray particles).
1. A₃X
2. A₁₃X₁₄
3. AX
4. A₂X₄
5. A₄X₄
Solution

Answer: C

Concept: Unit cells and equivalent atoms

Corners = 1/8
Edges = 1/4
Faces = 1/2
Inside = 1

Number of equivalent atoms

\[\begin{align*} \mathrm{eq.~A~particles} &= \dfrac{1}{4} ~\left ( \mathrm{12~edge~particles} \right ) + \left( \mathrm{1~particle~inside} \right ) \\[1.5ex] &= 4~\mathrm{A~particles} \\[2.5ex] \mathrm{eq.~X~particles} &= \dfrac{1}{8} ~\left ( \mathrm{8~corner~particles} \right ) + \dfrac{1}{2} ~\left ( \mathrm{6~face~particles} \right ) \\[1.5ex] &= 4~\mathrm{X~particles} \end{align*}\]

Determine empirical formula

\[\mathrm{A_4X_4} \longrightarrow \mathrm{AX}\]
Which of the following unit cells has the lowest packing density?
1. primitive cubic
2. body-centered cubic
3. face-centered cubic
Solution

Answer: A

Concept: Unit cells
A white granular solid melts at 880 °C. It does not conduct electricity unless it is dissolved in water. What type of crystalline solid is it?
1. molecular
2. ionic
3. covalent-network
4. metallic
5. none of these
Solution

Answer: B

Concept: Properties of solids
What is the coordination number for a simple cubic unit cell?
1. 3
2. 6
3. 8
4. 10
5. none of these
Solution

Answer: B

Concept: Properties of unit cells