• Support PF! Buy your school textbooks, materials and every day products Here!

Rates of Effusion

  • Thread starter Agent M27
  • Start date
  • #1
171
0

Homework Statement


So this isn't just one problem but it is a question as to why it is done one way for two instances and then the opposite way for a third problem. Here it goes.

1) An unknown gas effuses at a rate that is .462 times that of nitrogen gas (at the same temperature). Calculate the molar mass of the unknown gas in g/mol.

2) Uranium-235 can be seperated from U-238 by flourinating the uranium to form UF6 (which is a gas) and then taking advantage of the different effusion rates for compounds containing the two isotopes. Calculate the ratio of effusion rates for 238UF6 and 235UF6.

3) A sample of neon effuses from a container in 76 seconds. The same amount of an unknown noble gas requires 155 seconds. Identify the gas.

Homework Equations



[tex]\frac{RateA}{RateB}[/tex] = [tex]\sqrt{\frac{M_B}{M_A}}[/tex]

Where M = molar mass

The Attempt at a Solution



1) [tex]\frac{RateA}{RateB}[/tex] = .462

MA = [tex]\frac{M_B}{(.462)^2}[/tex]

MA = [tex]\frac{28.02}{(.462)^2}[/tex] = 131g


2) Rate A/ MA = U-238 RateB/ MB = U-235

[tex]\frac{RateA}{RateB}[/tex] = [tex]\sqrt{\frac{M_B}{M_A}}[/tex]

[tex]\sqrt{\frac{235.054}{238.051}}[/tex]= .9934

3) This is the one where I am confused on why it was carried out in the way as follows:
If I was to follow the format as shown above, where rateA is over rateB and then on the other side they switch positions, I arrive at a terribly incorrect answer:

[tex]\frac{76s}{155s}[/tex] = [tex]\sqrt{\frac{M_U}{20.18g}}[/tex]

.2404=[tex]\frac{M_U}{20.18}[/tex][tex]\Rightarrow[/tex] 4.85g which is the incorrect answer.

The only way I can arrive at the correct answer is when rate A is in the same postion on both sides for example:

[tex]\frac{76s}{155s}[/tex] = [tex]\sqrt{\frac{20.18g}{M_U}}[/tex]

MU = [tex]\frac{20.18g}{.2404}[/tex][tex]\Rightarrow[/tex] 83.9g

Which points to Krypton and that happens to be the correct answer. I realize this is a long question and I have the answer, but I want to make sure I am not missing something in the formatting and why it appears to change between problems. Thanks in advance.

Joe

Homework Statement





Homework Equations





The Attempt at a Solution

 

Answers and Replies

  • #2
epenguin
Homework Helper
Gold Member
3,632
701
You have just got momentarily confused into thinking the bigger number meant faster, but as it is time taken the bigger number means slower, so in your you first attempt you put it upside down. The ratio of atomic masses is close to 4 and Kr is clearly the answer they want.

(You might object that "effuses from a container in 76 seconds" is a pretty vague statement, but whatever it is it is the same for both and Kr is the answer they want. :biggrin:)
 

Related Threads for: Rates of Effusion

  • Last Post
Replies
2
Views
5K
  • Last Post
Replies
1
Views
2K
  • Last Post
Replies
4
Views
2K
Replies
8
Views
898
Replies
0
Views
2K
Replies
2
Views
14K
  • Last Post
Replies
2
Views
4K
Top