Two Questions

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joe001

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Hello,

I would appreciate any input on these two questions.

1. I came across this question: “What is the final temperature of 1 mol of gas that performs 50J of work adiabatically? The initial temperature is 0 Celsius. (Gas const = 8.314J/(mol*K))?”

They find the answer by doing PV=nRT, and get Tf = 6 Celsius. I’m wondering why they didn’t do -W = dU = n*cv*dT = 3/2 n*R*dT?


2. On one of the MCAT practice tests (AAMC MCAT Test 3R) in the Biological Sciences section #203 it asks:
“A man is treated with low doses of an estrogen analogue to destroy an estrogen-responsive adrenal tumor. Compared to an age-matched control (no estrogen treatment), this patient’s chances of developing osteoporosis will most likely be:
A. increased
B. decreased
C. approximately the same
D. approximately the same, but the disease will appear at an earlier age

The relevant passage says “the accelerated postmenopausal bone loss in women appears to be directly linked to estrogen deficiency, although the exact mechanism of this linkage is unknown”. (I can post the entire passage if needed)

The explanation says “Administration of estrogen to men would then have no therapeutic effect on bone loss, since they should not be suffering from withdrawal of the hormone, having never had high levels.” But just because they’re not suffering from low estrogen, doesn’t mean more estrogen would have no effect on them? More estrogen could help decrease the risk. Therefore, isn’t B. an equally plausible answer as C (correct answer)?

Thank you.

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1. I came across this question: “What is the final temperature of 1 mol of gas that performs 50J of work adiabatically? The initial temperature is 0 Celsius. (Gas const = 8.314J/(mol*K))?”

They find the answer by doing PV=nRT, and get Tf = 6 Celsius. I’m wondering why they didn’t do -W = dU = n*cv*dT = 3/2 n*R*dT?
They are wrong and you are right. The big red flag with their answer is that they have the temperature going up during a gas expansion with no heat flow (i.e. adiabatic).

An adiabatic expansion of an ideal gas results in a lower temperature because internal energy of the gas has decreased when some of its energy was used to do work. Their answer assumes the pressure remains constant during the expansion which it does not.
 
Thank you that was very clear!

What I also find confusing is enthalpy. Usually enthalpy = dU + d(PV), and we know dU = dQ - dW = dQ - V*dP - P*dV, so I wonder why doesn't enthalpy always equal dQ, since the rest would cancel out? But it's said enthalpy only equals dQ at constant pressure?

Would also still appreciate anyone's input on the second question. Thanks.
 
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