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Chloroform, CHCl3, has a vapour pressure of 120 mmHg at a certain temperature. Whatis the vapour pressure of a 0,200 m solution of a non-volatile, non-electrolyte solute inchloroform at the same temperature?

User Torsten Barthel
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1 Answer

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Step-by-step explanation:

Raoult's law states that the vapor pressure of a solvent that is a over a solution is equal to the vapor pressure of the pure solvent at the same temperature scaled by the mole fraction of the solvent.

Psolution = Xsolvent * P°solvent

Where Psolution is the vapor pressure of the solution, X is the molar fraction of the solvent and P°solvent is the vapor pressure of the pure solvent.

Our solvent is chloroform, we know its vapor pressure.

P°solvent = 120 mmHg

We still have to find the molar fraction of the solvent. The molar fraction of the solvent will be defined like:

Xsolvent = moles of solvent/(total number of moles)

We know that the concentration of the solution is 0.200 m. That means that we have 0.200 moles of solute in each kg of solvent.

In order to find the mole fraction we have to suppose that the solution has 1 kg of solvent.

mass of solvent = 1 kg

molality = 0.200 m

molality = moles of solute/(mass of solvent in kg)

0.200 m = moles of solute/1 kg

moles of solute = 0.200 m * 1 kg

moles of solute = 0.200 moles

So our solution has 0.200 moles of solute in 1 kg of solvent. The solvent is chloroform (CHCl₃). We can convert the mass of solvent into moles using the molar mass of it.

molar mass of CHCl₃ = 119.38 g/mol

mass of solvent = 1 kg = 1 kg * 1000 g/kg

mass of solvent = 1000 g

moles of solvent = 1000 g * 1 moles/(119.38 g)

moles of solvent = 8.377 moles

Our solution has 8.377 moles of solvent and 0.200 moles the non-volatile solute.

total number of moles = moles of solute + moles of solvent

total number of moles = 8.377 moles + 0.200 moles

total number of moles = 8.577 moles

Xsolvent = moles of solvent/(total number of moles)

Xsolvent = 8.377 moles/(8.577 moles)

Xsolvent = 0.977

Finally we found the mole fraction of the solvent and we can find the answer to our problem.

Psolution = Xsolvent * P°solvent

Psolution = 0.977 * 120 mmHg

Psolution = 117.2 mmHg

Answer: The vapor pressure of the solution is 117.2 mmHg.