The usual formula for calculating the vapor pressure of a solution is
(1) Psolution = Xsolvent P°solvent
where
Psolution is the vapor pressure of the solution,
Xsolvent is the mole fraction of the solvent, and
P°solvent is the vapor pressure of the pure solvent.
This can be rearranged to give the vapor pressure lowering, DP, which is P°solvent - Psolution :
(2) DP = Xsolute P°solvent
You can use either formula. In the second, the mole fraction of solute particles is
(3) Xsolute = moles of solute particles
moles of solute particles + moles of solvent
The solute (nonvolatile) is ethylene glycol, a molecular substance that gives only one "particle" when it is in solution. The solvent (volatile) is water. You need obtain the moles of each, starting with some arbitrary amount of solution, such as 100 mL.
Since this is a "50/50" mixture by volume, in 100 mL we will have 50 mL of ethylene glycol and 50 mL of water. Use the given densities to convert each volume to grams, and then convert to moles by dividing each by the respective molecular weight. Now you can now calculate Xsolute from equation (3).
P° is the vapor pressure of pure water at 100°C, which is one atmosphere, or 760 torr. You can use either pressure unit.
Now just plug into equation (2) to calculate DP. 
Steve
Free Forum Hosting
Important Announcement
First 