The common laboratory solvent benzene is often used to purify substances dissolv
ID: 523818 • Letter: T
Question
The common laboratory solvent benzene is often used to purify substances dissolved in it. The vapor pressure of benzene , C6H6, is73.03 mm Hg at 25 °C.
In a laboratory experiment, students synthesized a new compound and found that when 29.31 grams of the compound were dissolved in 276.8 grams of benzene, the vapor pressure of the solution was 71.50 mm Hg. The compound was also found to be nonvolatile and a non-electrolyte.
(benzene = C6H6= 78.12 g/mol)
What is the molecular weight of this compound?
Explanation / Answer
The vapor pressure of the solution is given by Raoult's Law:
Psolution = (P°solvent) (Xsolvent)
Xsolvent – mole fraction of solvent
P°solvent = the vapour pressure of the pure solvent
The mole fraction of benzene in this mixture is:
Psolution = (P°benzene) (Xbenzene)
Xbenzene = Psolution / P°benzene
= 71.50 mm / 73.03 mm
= 0.979
X1 = n1 / (n1 + n2 ) : n1 and n2 are the number of moles of solvent and solute
n1 / (n1 + n2) =
n = w/M
n1 = 276.8 grams / 78.12 g/mol = 3.54 mol
n1 / (n1 + n2) = 0.979
3.54 / (3.54 + n2) = 0.979
(3.54 + n2) 0.979 = 3.54
(3.54 + n2) = 3.62
n2 = 0.079 mol
w2/M2 = 0.079 mol: w2 is given = 29.31 grams
M2 = 29.31 grams/ 0.079 mol = 371 g/mol