Learning undergraduate engineering thermodynamics might be less painful with a blog. I hope that students, faculty and interested observers will share their thoughts on the laws of thermodynamics, phase and chemical equilibrium and many related topics.
Please post any questions or discussion related to this problem as comments on this message. Feel free to answer other students' questions. I will check the blog M-F and once on the weekend. Dr. B
6 comments:
Anonymous
said...
Hi prof, i'm still quite confused about the concept on lbmole. Is it just the unit for number of moles? will we be able to get mass by multiplying the quantity in lbmole by molecular weight?Thanks for the clarification.
How many significant figures didi you want us to include? I've always been taught that except in addition, the significant figures in the answer should match the least number of significant figures in the equation, but for the temperature problems, and for this problem, that seems inadequate. For example, if only one significant figure is used for the number of lbmole, then all the answers have only one significant figure.
Questioning: I agree with your comments about sig figs. I do not plan to stress about sig figs much in this course. We will focus on learning Thermo.
In general, 1 sig fig is not a good choice. In fact 2 is often too few for me to tell if you did the calculation correctly. So, in general, I would like you to carry many digits in all your calculations and then just round off your answer to 3 or maybe 4 sig figs. I will take off points for 1 sig fig and for 5 or more sig figs. I hope this is very simple and very clear.
I have not setup Moodle to accept HW uploads. So, instead please email your HW to the grader Ben. And since he is not really expecting it, you might want to send a copy to me.
Didn’t it take longer to do the HW in Excel ? I love Excel, but it can be time consuming. I am curious to see your work and to hear how long it took you to complete.
6 comments:
Hi prof, i'm still quite confused about the concept on lbmole. Is it just the unit for number of moles? will we be able to get mass by multiplying the quantity in lbmole by molecular weight?Thanks for the clarification.
kik :
Do you understand regular old moles ?
If so consider calling them gram-moles or gmoles.
gmoles = grams / (MW in g/gmole)
Similarly in the AE system...
lbmoles = lbm / (MW in lbm/lbmole)
In either system, a mole is a fixed (very large) number of molecules. A lbmole has MORE molecules in it becuase 1 lbm is more mass than 1 gram.
If you are still confused, come to my office hours and I will try to draw something that will make this more clear.
How many significant figures didi you want us to include? I've always been taught that except in addition, the significant figures in the answer should match the least number of significant figures in the equation, but for the temperature problems, and for this problem, that seems inadequate. For example, if only one significant figure is used for the number of lbmole, then all the answers have only one significant figure.
Questioning:
I agree with your comments about sig figs. I do not plan to stress about sig figs much in this course. We will focus on learning Thermo.
In general, 1 sig fig is not a good choice. In fact 2 is often too few for me to tell if you did the calculation correctly. So, in general, I would like you to carry many digits in all your calculations and then just round off your answer to 3 or maybe 4 sig figs. I will take off points for 1 sig fig and for 5 or more sig figs. I hope this is very simple and very clear.
Dr. B
I did my homework in excel, you said we could post it somewhere but where agian? thanks
Mei:
I have not setup Moodle to accept HW uploads. So, instead please email your HW to the grader Ben. And since he is not really expecting it, you might want to send a copy to me.
Didn’t it take longer to do the HW in Excel ? I love Excel, but it can be time consuming. I am curious to see your work and to hear how long it took you to complete.
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