AAMC 4 question 20 on PS

[kfsa1]

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Hi guys,
I was wondering about the explanation behind question 20 on AAMC 4 which is not the CBT.

The question asks about how the accuracy of determining O2 concentration would be if another gas was added.

I understand that bc the milivolt recorder can't determine between gases, the results would be skewed with the addition of another gas. However, what about the answer that states the partial pressure of O2 would be decreased? I always thought addition of another gas would take away from the volume that could be used by the original gas which = lower partial pressure of the original gas....or is this only in the situation where there is a set amount of pressure that could exist in a system which then must be divided among all the gases in the system?

 

[movax]

I just did this CBT last night, and I said it was D.

Question stem says that the second gas will "be reduced in a reaction analogous to that of oxygen". This means that both oxygen and gas #2 will be reduced at the Ag electrode, which is the cathode. This will have an immediate and very direct effect on the current the recorder sees (it's essentially using the resistor as a current shunt, and correlating measure voltage across the resistor to current).

Hence, the accuracy goes down, because this method has no idea what gas is doing what. All it sees is a change in current. D is the best answer. C would be true, I think, but it's not why the accuracy of the system goes down.

 

[kfsa1]

Right. The reason why I thought C could be a possible answer is because it is stated in the passage that oxygen concentration is proportional to the potential. If the partial pressure of O2 is reduced, that is in essence a decrease in concentration of O2. This is what is confusing me.

Maybe I am not getting the concept of partial pressures correct. My understanding was that if for example there was a finite amount of pressure in a system, the partial pressures of all the gases in that system total to that finite pressure. E.G O2 + H2 + N2 = 1atm and the partial pressures = molar fraction. IF however, we can add on extra gases without the need to total to a finite pressure, I would see this making sense. Another gas adding on will increase the total pressure of the system which essentially reduces the molar fraction = lower partial pressure. BUT, the concentration of the O2 does not decrease.

BTW, where in michigan are you? I'm in ann arbor.

 

[movax]

Maybe I am not getting the concept of partial pressures correct. My understanding was that if for example there was a finite amount of pressure in a system, the partial pressures of all the gases in that system total to that finite pressure. E.G O2 + H2 + N2 = 1atm and the partial pressures = molar fraction. IF however, we can add on extra gases without the need to total to a finite pressure, I would see this making sense. Another gas adding on will increase the total pressure of the system which essentially reduces the molar fraction = lower partial pressure. BUT, the concentration of the O2 does not decrease.

I think you're right with your understanding of partial pressures; IMHO this one of the questions where the answer happens to be "more right' than the other one.

If the partial pressure of oxygen changed with the addition of another gas that wasn't reduced (so let's say N2, I think that is pretty inert), the oxygen will still get reduced at the electrode, and you should see the same current you'd see with pure oxygen. I believe the passage is trying to drill home the fact that it's the reduction of the gas (in this case oxygen) that generates the current, regardless of its partial pressure.

BTW, where in michigan are you? I'm in ann arbor.

Me too, I live and work in Ann Arbor at the moment.

 

[ModusProbandi]

Me too, I live and work in Ann Arbor at the moment.

Me too!