Question: Questions 31 and 35 in the morning session of the first day of the 2014 Chemical Engineering professional examination
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31. What is the value of the degree of freedom for the gas-liquid equilibrium system of a binary material solution? (A)2 (B)3 (C)4 (D)1 35. At 20°C, the vapor pressures of methanol and ethanol are 12.5 KPa and 5.9 KPa respectively. These two substances can be considered to obey Raoult’s law over a wide range of compositions. Therefore, what is the vapor pressure (in KPa) of a mixture composed of 50 kg of methanol and 100 kg of ethanol at 20°C? (A)10.30 (B)9.74 (C)8.66 (D)8.10
This post was last edited by ydm4585 on 2015-6-3 10:34. At constant temperature and pressure: 2-2+2=2. First, calculate the molar fractions of each component in the mixture; then the result is obtained by multiplying X1 by 12.5 and X2 by 5.9. No further detailed calculation is needed. Constant temperature and constant pressure; the last number is 2, which is incorrect
For question 31, the degree of freedom is 2, right? Determining two states in tpxy is enough to fix everything
What is the value of the degree of freedom for the gas-liquid equilibrium system of a binary material solution? (A) At 20°C, the vapor pressures of methanol and ethanol are 12.5 KPa and 5.9 KPa respectively. These two substances can be considered to obey Raoult’s law over a wide range of composition values. Therefore, what is the vapor pressure (in KPa) of a mixture composed of 50 kg of methanol and 100 kg of ethanol at 20°C? (C)8.66
8.10 Choose D 12.5*0.3333+5.9*0.6666
Ugh, what’s needed is the molar fraction, not the mass fraction. That’s how it should be understood; factors such as concentration are influenced by the number of molecules, that is, by moles, rather than by their weight, that is, mass.
Haha, yes, it’s my fault. I was looking too quickly; I’m starving. Sorry about that
Choose C, 8.66, determined based on the molar ratio! :P
Oh, after reading the post, I realized that’s where my problem lay :)
I have a question: where does this 2-2+2 come from?