6 replies to this topic

[Mr. Molecule]

The question: what would be the smallest volume of a closed cylinder to completely burn 0.25 grams of ethyl alcohol if 100% pure oxygen gas at 300 psi is present and is the only other material in the cylinder.

 

Three molecules of 02 are needed for every molecule of ethyl alcohol burnt. 

 

In 0.25 grams of Ethyl there are 0.32 x 10 to the twenty-third molecules.

 

There are (if I am reading the chart correctly) 1.59 x 10 to the nineteenth molecules of 02 in one cubic centimeter at 300 psi.

 

Either I am working with incorrect data or messing up on my scientific notation, my answer is an unrealistically large volume.

 

Thanks for your help on this one.

[MrShorty]

I get different results for the number of molecules of EtOH in 0.25 g (by about a factor of 10). Assuming ideal gas, I also get a different density (molecules/cc) for O2 (by about a factor of 30). Potentially, our results would be different by as much as a factor of 300. Review your calculations for converting mass of EtOH to moles/molecules EtOH as well as your density of O2.

[latexman]

Don’t forget, there could be more moles of gases on the product side of the equation:

C2H5OH + 3 O2 —> 2 CO2 + 3 H2O

But it gets complicated. Initially, if it’s hot enough the water may be vapor, but it could cool and the water will condense to liquid.

[Mr. Molecule]

MrShorty,

 

Thank you. I was hoping for a more straight forward answer but, you are right, moles are where I left

reason for easy, wishful thinking.

 

I was hoping for a simple savior who's answer was:

 

The cc's  of O2 @ 300psi required, not including the .25 gr of ethyl is, within the closed, purified, container, is:

[Bobby Strain]

You need to work at a larger scale for your calculations. Like gram-moles, not molecules. Latexman probably led you astray. Pay attention to the question; don't make it unnecessarily complicated.

 

Bobby

[MrShorty]

I understand. Over the years I've participated on this forum, we have generally been reticent to just hand out answers to students' homework questions. We have generally preferred to collaborate with the student as they discover the answers for themselves. If one of your expectations from this group is to have people give you answers to your homework questions, I suspect you will be disappointed with us.

 

I think you will find, though, that, if you will share your work (for example, explain how you came up with 0.32E23 molecules of EtOH in 0.25 grams) in enough detail that we can see what you are trying to do, we will be quite happy to help you see where you are in error and help you understand the correct way to perform that step.

 

[latexman]

The question: what would be the smallest volume of a closed cylinder to completely burn 0.25 grams of ethyl alcohol if 100% pure oxygen gas at 300 psi is present and is the only other material in the cylinder.

The cc's of O2 @ 300psi required, not including the .25 gr of ethyl is, within the closed, purified, container, is:

The second adds some clarity, but . . . what’s the temperature? A gas volume without a T and a P is meaningless.