Quantities such as the isothermal compressibility help us to determine changes in the thermodynamic state functions for "real substances". These quantities can be measured through direct laboratory measurements, or through the use of substance-specific equations of state, such as the van der Waals equation.

Chemistry: The Molecular Science
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Author:John W. Moore, Conrad L. Stanitski
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Chapter9: Liquids, Solids, And Materials
Section: Chapter Questions
Problem 45QRT: At the critical point for carbon dioxide, the substance is very far from being an ideal gas. Prove...
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I am pretty sure I have done part a, b, and need a limit for C but I can't see how it reduces to 1/p, also need help on the rest of them thank you.

3. Quantities such as the isothermal compressibility help us to determine changes in the
thermodynamic state functions for "real substances". These quantities can be measured
through direct laboratory measurements, or through the use of substance-specific
equations of state, such as the van der Waals equation.
RT
a
2
Vm
Vm²
(a) Work through the math to obtain a formula for the isothermal compressibility of a van
der Waals gas.
P =
KT ==
- b
1 /0Vm
Vm Op T
[Hint: this will probably be significantly easier if you use the reciprocal relationship for partial derivatives!
This way, you can work from the van der Waals equation as given above, with P as the dependent
variable. The resulting derivative is not necessarily “simple”, but you should be able to navigate through
with some judicious use of the chain rule, and a bit of algebraic maneuvering.]
(b) Once you have obtained your result, demonstrate that the units work out correctly.
(c) Next, demonstrate that your expression reduces to the ideal gas result (KT = 1) at
large values of Vm.
(d) Calculate the value of KT for a hypothetical ideal gas at T = 525 K and Vm =
2.50 L mol-¹, and again at T = 525 K and Vm = 25.0 L mol-¹.
(e) Calculate the value of KT for ethanol (as a van der Waals gas, use parameters from E & R
table 7.4) when T = 525 K and Vm = 2.50 L mol-¹, and again at T = 525 K and Vm =
25.0 L mol-¹.
Transcribed Image Text:3. Quantities such as the isothermal compressibility help us to determine changes in the thermodynamic state functions for "real substances". These quantities can be measured through direct laboratory measurements, or through the use of substance-specific equations of state, such as the van der Waals equation. RT a 2 Vm Vm² (a) Work through the math to obtain a formula for the isothermal compressibility of a van der Waals gas. P = KT == - b 1 /0Vm Vm Op T [Hint: this will probably be significantly easier if you use the reciprocal relationship for partial derivatives! This way, you can work from the van der Waals equation as given above, with P as the dependent variable. The resulting derivative is not necessarily “simple”, but you should be able to navigate through with some judicious use of the chain rule, and a bit of algebraic maneuvering.] (b) Once you have obtained your result, demonstrate that the units work out correctly. (c) Next, demonstrate that your expression reduces to the ideal gas result (KT = 1) at large values of Vm. (d) Calculate the value of KT for a hypothetical ideal gas at T = 525 K and Vm = 2.50 L mol-¹, and again at T = 525 K and Vm = 25.0 L mol-¹. (e) Calculate the value of KT for ethanol (as a van der Waals gas, use parameters from E & R table 7.4) when T = 525 K and Vm = 2.50 L mol-¹, and again at T = 525 K and Vm = 25.0 L mol-¹.
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I follow your solution until you solve for the partial, when I do it I am left with a 3PVm and -2RT expression on my solution, where did they go? You accounted for them in the full derivative but then when doing the algebra they seem to disappear for no reason. Can you explain "Step 3" (at least on here, probably page 3) better?

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