Answer:
82.5 mmHg
Explanation:
The vapor pressure of a solution that contains a non-volatile solute will depend on the mole fraction of the solvent and on the vapor pressure of the pure solvent at the same temperature.
P
sol= χsolvent
⋅ P∘solvent, where
Psol - the vapor pressure of the solution
χ
solvent - the mole fraction of the solvent
P
∘solvent - the vapor pressure of the pure solvent
Now, the most important thing to realize here is that lithium fluoride,
LiF
, a soluble ionic compound, will dissociate completely in aqueous solution to form lithium cations and fluoride anions.
LiF
(aq]
→ Li+(aq] + F
− (aq]
Notice that every mole of lithium fluoride produces one mole of lithium cations and one mole of fluoride anions.
This means that you get a total of two moles of ions for every one mole of lithium fluoride in the solution.
Use lithium fluoride's molar mass to determine how many moles would be found in the given sample
78.4g⋅ (1 mole LiF
/25.94
g) = 3.022 moles LiF
This means that the solution will contain
3.022 moles LiF ⋅ (2 moles ions
/1 mole LiF) = 6.044 moles ions
Now, water's mole fraction in this solution will be equal to the number of moles of water divided by the total number of moles present in the solution.
To get the number of moles of water, use its molar mass
253 g ⋅ (1 mole H
2O
/ 18.015
g) = 14.044 moles H
2O
This means that water's mole fraction will be
χwater = 14.044 moles / (14.044
+6.044
)moles
= 0.6991
The vapor pressure of the solution will thus be
Psol= 0.6991 ⋅ 118 mmHg = 82.5 mmHg
