Posted by Lauren on .
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,
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,
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,
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,
we are the same person LOL

Chemistry 
Jen/Lauren,
***We are siblings haha

Chemistry 
DrBob222,
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 1.0000.8828 = 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,
Thanks DrBob222! You are the best! :]

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

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