EK 1001 Chemistry questions help.

[solo-king]

1) Which of the following affects the average force (per unit area) exerted by a gas on the wall of its container?

I. The average speed of a gas molecule
II. The frequency of collisions between gas molecules and the wall
III. The volume of a gas molecule

A. I only
B. II only
C. I and II only
D. I,II, and III

2) Suppose the reaction:

CO(g) + Cl2(g) = COCl2(g)

has an equilibrium constant of 530. What is the equilibrium constant of the reaction:

2CO(g) + 2Cl(g) = 2COCl2(g)

A. 530
B. 1060
C. 2120
D. 2.81 x 10^5

Please i'm looking for an explanation for the answers in both questions because EK 1001 assumes you know this stuff and barely gives an explanation if any at all in their 1001 series. Thank you!

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[lifetothefullest]

Force per unit area is also known as pressure. So if a gas particle strikes a wall faster, do you think that'll exert more, less, or the same pressure? Same question but now with frequency of collisions and volume of gas molecule.

From what you know about equilibrium constants, is it an intensive or extensive property? That is, does it change based on how much stuff you started with?

 

[solo-king]

Force per unit area is also known as pressure. So if a gas particle strikes a wall faster, do you think that'll exert more, less, or the same pressure? Same question but now with frequency of collisions and volume of gas molecule.

From what you know about equilibrium constants, is it an intensive or extensive property? That is, does it change based on how much stuff you started with?

Thanks for the response. I know force per area is pressure so with that in mind, i assumed all the factors would affect the gas particle for instance, increasing the average speed will increase the pressure of the gas molecules due to more collisions. Also an increase in speed most likely means an increase in the temperature which increases K.E and from the ideal gas law, pressure and temperature are directly proportional to each other. Same goes for the volume, volume and pressure are indirectly proportional to each other. I picked D as the answer but it was C and i fail to see how volume has no affect on the pressure of a gas.

For number two , i assumed the keq is only affected by temperature and unaffected by number moles, pressure, volume etc. SO increasing the concentration of the reaction should have no effect on the keq right? Thanks.

 

[lifetothefullest]

Your intuition for the first is correct and your reasoning is logical. However, remember that they are talking about the volume of the gas particles themselves, not the volume of the whole gas. So imagine the space occupied by an Ar molecule versus the box occupied by a mole of Ar gas. The volume of the gas molecule is what it is, you can't change it. So it has no effect on the pressure exerted by the gas. However, there is another interpretation of the question which would give the answer you got - that is, assuming that you can vary the volume of the gas particle (e.g. using Ar instead of Ne). Then, if you assume, say, a van der Waals equation of state, then pressure of the gas does change. But given their answer, that's not the correct interpretation (also given that the MCAT tests general chemistry and organic, not thermodynamics so I doubt you'll need to know the van der Waals equation of state - unless it was taught in general chemistry). So basically, I believe they want you to assume that the gas is ideal.

For the second one, you are correct again. K is only dependent on temperature and so altering the concentration of your starting materials is not going to give you a different K. For example, say your K = 2. You'll end up with a 2:1 ratio of products to reactant (singular) no matter what numbers you use. 1 mol/0.5 mol is the same as 2 mol/1 mol even though you started with more stuff in the latter case.

Hope this helps.

 

[sazerac]

For the second question, remember the definition of K is products raised to their coefficients divided by reactants raised to their coefficients. So you are looking at C^2 / (A^2 B^2).

Thankfully they already told you what C / (A B) is. Thus the answer is 530^2, which could only be the last answer.

This is a very MCAT question: seeing if you can go back to basic principles in an unfamiliar setting, and can you quickly throw out 3 bad answers.