Examkracker Chemistry Lecture 2 Practice Questions #27 and #29

[MedGrl@2022]

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Hey SDNers,

I am studying for the MCAT and I had a couple questions from Chemistry Lecture 2 Examkracker Practice Problems.

#27. Ammonia burns in air to form nitrogen dioxide and water.

4NH3(g) + 7O2(g) --> 4NO2(g) + 6H2O(l)

If 8 moles of NH3 are reacted with 14 moles of O2 in a rigid container with an initial pressure of 11atm, what is the partial pressure of NO2 when the reaction runs to completion? (Assume constant temperature.)

A. 4 atm
B. 6 atm
C. 11 atm
D. 12 atm

My answer. C.

Examkracker answer: A is correct. The number of moles of gas is extra information. If the container began at 11 atm then each gas is contributing a pressure in accordance with its stoichiometric coefficient. When the reaction runs to completion, the only gas in the container is nitrogen dioxide, so the partial pressure of nitrogen dioxide is the total pressure. The volume of the container remains constant, so the pressure is in accordance with the stoichiometric coefficient of nitrogen dioxide.

so shouldn't the answer be C and not A?

#29. A force is applied to a container of gas reducing its volume by half. The temperature of the gas:

A. decreases
B. increases
C. remains constant
D. The temperature change depends upon the amount of the force used.

My answer C. my reasoning is because the pressure increased due to this force so although the volume decreased by 1/2 the pressure increased by 1/2 thus nRT should remain constant

Examkracker answer: B is correct. The force does work on the gas, which means that the internal energy of the gas is increased. Since the internal energy of the gas is increased, and the number of moles remains the same, the temperature, which is average kinetic energy per mole, also increases.

I understand EKs answer but how am I supposed to differentiate between increased pressure and force for these types of problems.

Thank you for all your help.

Sincerely,

Verónica

 

[maybemed2013]

Number 27.

The total mole of reactions is 22 so 22 moles is equal to 11 atm.
The total mole on the product side is 8.

You can solve this problem with ration
22:11
8:X

As you can see the amount of moles is double of the atm so the answer is 4 atm.

Number 29.

This is a relationship that you should know.
V is inverse proportional to T. Think about it if you are in a small area with # of people you will feel hotter than being in a big area with the same amount of people.

 

[DPTinthemaking15]

Sorry for bringing this thread back up from the dead, but I am confused on number 29. I am using BR for content review and it says "Volume is directly proportional to temperature, assuming that pressure is kept constant." I'm assuming whenever force is being applied, it would violate Charles Law?

 

[Tigriski]

Sorry for bringing this thread back up from the dead, but I am confused on number 29. I am using BR for content review and it says "Volume is directly proportional to temperature, assuming that pressure is kept constant." I'm assuming whenever force is being applied, it would violate Charles Law?

even though the answer for #29 mentions force.... that seems like an obvious thing to mention. To compress anything you need a force to make this happen. just think about what we know about living in the real world. So intuitively we know that an increase in pressure in the chamber thats being compressed comes from the fact that the molecules in that chamber are moving quicker and therefore bouncing off the edges of the chamber more often (this is what we measure as pressure; more molecules hitting the edge of the chamber more energy transferred, in a sense, the greater the pressure). sticking withe the faster moving molecules thought; as molecules increase in speed, so does the energy. Now think about phase changes (solid, liquid, gas) as temp increases so does the speed of these molecules.

now if you put this all together: faster moving molecules, means more energy, means higher temps.

In my mind this question isnt truly about forces.... trying to think intuitively really helps with this.

the force is directly about changing the volume.....not pressure. well kind of? I think I understand what youre talking about when you say "violating charles Law" but i dont think this problem is trying to talk about force in terms of "force of pressure" like. it's implying the force of whatever is acting on the chamber is not defined in the system

 

[desperadoflakes]

Deleted

 

[desperadoflakes]

#27)

You start with 22 moles of a gas, and if the reaction runs to completion you make 8 moles of a gas (and 12 moles of H2O, but we don't care about that for the purposes of this question. The thing that you have to remember about gas - 1 mole of a gas at a set temperature and a set pressure has a SET volume, REGARDLESS of what the gas is. Especially if we're making ideal gas law assumptions. Remember: 1 mole of an ideal gas at STP has a volume of 22.4L.

Now, from this you can deduce that the same logic applies if a gas has a set temperature and a set volume. The question tells you that this reaction takes place in a rigid container, which means that the volume IS set (it also tells you the temp is constant).

So if 22 moles of a gas have a pressure of 11 atm, that means that 1 mole of a gas at that same temperature and pressure will have 0.5atm. And 8 moles of a gas will be 4 atm.

#29)

This is tricky, because we are dealing with three variables, and usually in these questions they will give you one of them and ask you to relate the other two. However, they didn't give you one here, but it is tempting to answer the question as if they did. Remember the combined gas law:

PV/nT = PV/nT ; it is safe to assume that n remains constant (no chemical reaction takes place). The question tells you that volume is decreased. We know that when the volume decreases the pressure increases, this is obvious. The tricky part is realizing that the temperature also increases (presumably because the pressure increases to a value greater than twice the original pressure). This equation doesn't really lead us to what we want, so let's try this one:

U = Q + W (W = work done on the system)
Because there is no transfer of heat, this is an adiabatic process and Q = 0. So U = W

We know that temperature is a measure of the average kinetic energy of a system, and that sounds a lot like U, so for our purposes it makes sense to think of U as being directly representative of temperature.

Because work is being done ON the system, we know that W > 0, so there must be an increase in temperature. We also know that the pressure must increase because W = P∆V, and we are reducing V by half. We know that P can't just increase by a factor of 2, because that would mean no change in W, so no work. So pressure increases by a factor greater than 2, and temperature also increases due to an increase in internal energy.