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Homework Help: Chemistry

Posted by Lauren on Saturday, February 18, 2012 at 10:24pm.

A solution is made by mixing 30.0 mL of ethanol, C2H6O, and 70.0 mL of water. Assuming ideal behavior, what is the vapor pressure of the solution at 20 °C?

I got an answer of 15.45 torr and it marked me incorrect and said "Your answer is the the partial vapor pressure contributed by the water component of the solution. Since ethanol is a volatile liquid, it will also contribute to the vapor pressure."

HELP PLEASE!

• Chemistry - DrBob222, Saturday, February 18, 2012 at 10:37pm

  You have half the answer. Just finish it. You didn't provide data (densities, vapor pressure) etc so I can't use numbers. I assume the part you have is
  pH2O = XH2O*P^o_{normal v.p. H2O}
  You need to also do
  
  pEtOH = XEtOH*P^o_{normal v.p. EtOH}
  Then Ptotal = pH2O + pEtOH
  

• Chemistry - Jen, Saturday, February 18, 2012 at 10:40pm

  Sorry here are the data:
  
  Values at 20 degrees Celcius
  
  ethanol : density (g/mL)= 0.789/ vapor pressure (torr)= 43.9
  
  water: density (g/mL) =0.998/ vapor pressure (torr)= 17.5
  
  
  I' m still confused on what you are telling me to do :( Can u please clarify...
  

• Chemistry - Lauren, Saturday, February 18, 2012 at 10:41pm

  Sorry here are the data:
  
  Values at 20 degrees Celcius
  
  ethanol : density (g/mL)= 0.789/ vapor pressure (torr)= 43.9
  
  water: density (g/mL) =0.998/ vapor pressure (torr)= 17.5
  
  
  I' m still confused on what you are telling me to do :( Can u please clarify...
  

• Chemistry - Jen/Lauren, Saturday, February 18, 2012 at 10:41pm

  we are the same person LOL
  

• Chemistry - Jen/Lauren, Saturday, February 18, 2012 at 10:42pm

  ***We are siblings haha
  

• Chemistry - DrBob222, Saturday, February 18, 2012 at 11:18pm

  The total vapor pressure of the solution is the partial pressure of the H2O + the partial pressure of the EtOH. I calculated the pH2O and obtained 15.45 torr just as you did. Therefore, you must have used the right mole fraction of H2O. I calculated that to be 0.8828 (I know that's too many places but I can round later). Look in your notes. If that is true than 1.000-0.8828 = 0.11715 for mole fraction EtOH or you can calculate the mole fraction of EtOH from scratch.
  Then pEtOH = XEtOH x 43.9 = approximately 5 torr.
  So the total pressure will be 15 from the H2O + 5 from the EtOH (of course you know these are not ideal solutions) = about 20 torr.
  You need to go back through and come up with better numbers than those I've used. Also remember that you can't have more than 3 s.f. so don't report a 15.45 (or course that will be changed now) since that has 4 s.f.
  

• Chemistry - Kylie, Saturday, February 18, 2012 at 11:24pm

  Thanks DrBob222! You are the best! :]
  

• Chemistry - Kylie, Saturday, February 18, 2012 at 11:25pm

  I had this exact problem for my homework but different numbers. I got it right thanks
  

• Chemistry - Jen, Saturday, February 18, 2012 at 11:26pm

  Thanks sooo much DrBob222! And I'm glad to help kylie :D
  

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