Intermolecular Forces and Properties

4.a. In terms of the types and relative strengths of all the intermolecular forces in each compound, explain why the boiling point of $CS_2(l)$ is higher than that of $COS(l)$.

4.b. A 10.0 g sample of $CS_2(l)$ is put in an evacuated 5.0 L rigid container. The container is sealed and heated to 325 K, at which temperature all of the $CS_2(l)$ has vaporized. What is the pressure in the container once all of the $CS_2(l)$ has vaporized?

6.a) The student tests the electrical conductivity of each solid and observes that neither solid conducts electricity. Describe the structures of the solids that account for their inability to conduct electricity. (1 point)

The solids are ionic compounds. The ions are held in fixed positions in the solid lattice and are not free to move, so they cannot conduct electricity.

6.b) The student places excess $CaSO_4(s)$ in a beaker containing 100 mL of water and places excess $PbSO_4(s)$ in another beaker containing 100 mL of water. The student stirs the contents of the beakers and then measures the electrical conductivity of the solution in each beaker. The student observes that the conductivity of the solution in the beaker containing the $CaSO_4(s)$ is higher than the conductivity of the solution in the beaker containing the $PbSO_4(s)$.

Which compound is more soluble in water, <math-inline>CaSO\_4(s)</math-inline> or <math-inline>PbSO\_4(s)</math-inline>? Justify your answer based on the results of the conductivity test. (1 point)

<math-inline>CaSO\_4(s)</math-inline> is more soluble in water. Since the conductivity of the <math-inline>CaSO\_4</math-inline> solution is higher, it means that more ions are dissolved in the solution, indicating that it is more soluble.

6.c) The left side of the diagram below shows a particulate representation of the contents of the beaker containing the $CaSO_4(s)$ from the solution conductivity experiment.

![](https://zustagstorage.blob.core.windows.net/community/files_questions/frq/ap24-frq-chemistry_e99e1403d8554e88946f80f876cef6a2)

Draw a particulate representation of <math-inline>PbSO\_4(s)</math-inline> and the ions dissolved in the solution in the beaker on the right in the diagram. Draw the particles to look like those shown to the right of the beaker. Draw an appropriate number of dissolved ions relative to the number of dissolved ions in the beaker on the left. (2 points)

![](https://zustagstorage.blob.core.windows.net/community/files_questions/frq/ap24-frq-chemistry_e99e1403d8554e88946f80f876cef6a2)

6.d) The student attempts to increase the solubility of $CaSO_4(s)$ by adding 10.0 mL of 2 $M$ $H_2SO_4(aq)$ to the beaker, and observes that additional precipitate forms in the beaker. Explain this observation. (1 point)

The addition of <math-inline>H\_2SO\_4</math-inline> increases the concentration of <math-inline>SO\_4^{2-}</math-inline> in the solution. According to Le Chatelier's principle, the equilibrium will shift to the left, causing more <math-inline>CaSO\_4</math-inline> to precipitate out of solution.

4.a. Calculate the number of moles of $NH_2Cl$ (molar mass 51.48 g/mol) present in 1.0 L of a solution in which the concentration of $NH_2Cl$ is 0.0016 g/L.

$\frac{0.0016 , g}{L} \times \frac{1 , mol}{51.48 , g} \times 1 , L = 3.1 \times 10^{-5} , mol$

4.b. $NH_2Cl$ is highly soluble in water, whereas $NCl_3$ is nearly insoluble. Explain this observation in terms of the types and relative strengths of the intermolecular forces between each of the solutes and water.

$NH_2Cl$ is polar and can form hydrogen bonds with water molecules, which are strong intermolecular forces. $NCl_3$ is only slightly polar and cannot form hydrogen bonds with water, so the intermolecular forces between $NCl_3$ and water are weak.

4.c. The value of $\Delta H_{vaporization}$ for $NCl_3(l)$ is 32.9 kJ/mol. Calculate the amount of energy required to vaporize a 15.0 g sample of $NCl_3$ (molar mass 120.36 g/mol).

$15.0 , g , NCl_3 \times \frac{1 , mol , NCl_3}{120.36 , g , NCl_3} \times \frac{32.9 , kJ}{1 , mol , NCl_3} = 4.10 , kJ$

6.a) In the following table, list all of the types of intermolecular forces present in pure samples of HBr($l$) and HF($l$).

6.b.i) Based on the types and relative strengths of intermolecular forces, explain why ΔH°$_{vap}$ of HF($l$) is greater than that of HBr($l$). 6.b.ii) Calculate the amount of thermal energy, in kJ, required to vaporize 6.85 g of HF($l$).

6.c) Based on the arrangement of electrons in the Br and F atoms, explain why the bond length in an HBr molecule is greater than that in an HF molecule.