FE Exam Review - FE Environmental - Air Quality - Mass Balance

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[@kiquedc]

Hi! Why did your Ciss change from 0.0787 to 0.787 when solving for Ci? Was it a conversion factor?

[@directhubfeexam]

Totally messed it up, sorry about that. Ciss = 0.0787 not 0.787 - I wrote that mistakenly. The final answer should be: 0.0787 mg/m3

[@kiquedc]

@@directhubfeexam Thank you for explaining it! When using the correct Ciss though, the final answer for Ci comes to 0.0783 which is not in options.

[@directhubfeexam]

@@kiquedc yeah sorry about that, it’s a bad example that needs to be redone.

[@kiquedc]

@@directhubfeexam alright! Thanks for replying and making FE Environmental content! 🙏🏼

[@directhubfeexam]

@@kiquedc Thank you for watching!

[@loveh6094]

Great video so helpful for my studying THANK YOU!

[@directhubfeexam]

Thank you! I wish I had not made the mistake other caught in the comments. I’m glad the video helped you grasp the concept!

[@tigesttefera3658]

Hi, thanks for your videos it is so helpful and concentrates on unique question. I just have one question, Ciss shouldn't be 0.079, so shouldn't we use that in ci eqn and find ci= 0.0786?

[@jasneetsingh6926]

My final answer was 0.0783 for Ci because Ciss was equal to 0.0787, is this correct? Thanks.

[@TheLychie]

I got Ciss=0.0789 without rounding most of my numbers (I think). My answer ended up being Ci = 0.0785.
I think he should change answer A to something less close to the answer to account rounding/small error.

[@Step_19313]

I don't understand. I see that you used 0.787 instead of 0.0787, but using 0.787 does not give you any of the available answers

[@andrewgrimes1771]

If it states in the problem that it is a first order equation what is the point of spending the time to write out the simplified first order equation before just solving for Ci?

[@directhubfeexam]

are you referring to the dci/dt differential equation? Yeah that's actually not important if you are taking the FE exam since everything you need is provided in the handbook. But I noticed there is no other video for this topic on youtube so it was intended to help those also not taking the FE exam. Obviously at university, the class work will likely make us solve for the differential equation.

[@connorbannochie9014]

Very helpful thank you!

[@directhubfeexam]

You're welcome, this was one of my favorite problem and video to make!!

[@StrawberryL0verx3]

hi, where did you get 0.787 mg/m^3 for the Ci(3) equation? Didn't you just calculated and got 0.0787 mg/m^3?

[@directhubfeexam]

Yeah I’m sorry, I totally wrote the wrong value in the calculation for Ciss. It should be the value calculated of 0.0787 .

[@pinkie224]

So can you solve for S and then go straight to the Ci equation in the handbook instead of using the material balance equation?

[@directhubfeexam]

Hi David, yes you can! If they ask you to find the indoor concentration (typically the case) that's all you do. Note, this Ci equation is for non-steady state. The hint it's a non-steady state condition is that they give us a time ("after 3 hours).
If they did not give us a time and said "uniform concentration throughout the room" then you would use the Ciss equation to find the indoor concentration.