AP Chemistry Question of the Day --12

Here comes the answer to question 11 I posted yesterday. The correct answer is C. 

This is a question about intermolecular forces as I discussed earlier in one of my blogs regarding the prediction of physical properties.

When the pressure of the gas is increased and the gas is about to condense, the gas can no longer deemed as ideal gas. At this point, the intermolecular forces between the gas molecules are no longer negligible. The stronger the intermolecular forces are, the less pressure is needed for the gas to condense.

All  gases A, B, and  C are non polar covalent bonded molecules. The only intermolecular force is London dispersion force. London dispersion force is caused by the instantaneous dipoles as molecules move around. The more complex the electronic structure of the molecule, the stronger the London dispersion force. Normally in a series of compound with similar structures, the higher the molar mass, the stronger the London dispersion force. As a result, answer C is the right answer.

Following is Question 12 of the series. 

12. PCl5(g) decomposes into PCl3(g) and Cl2(g) according to the equation above. A pure sample of PCl5(g) is placed in a rigid, evacuated 1.00 L container. The initial pressure of the PCl5(g) is 1.00 atm. The temperature is held constant until the PCl5(g) reaches equilibrium with its decomposition products. The figures below show the initial and equilibrium conditions of the system. Which of the following is the most likely cause for the increase in pressure observed in the container as the reaction reaches equilibrium?

(A) A decrease in the strength of intermolecular attractions among molecules in the flask
(B) An increase in the strength of intermolecular attractions among molecules in the flask
(C) An increase in the number of molecules, which increases the frequency of collisions with the walls of the container
(D) An increase in the speed of the molecules that then collide with the walls of the container with greater force

I will post the answer and explanation tomorrow.

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