Question

The vapor pressure of chloroform (CHCl3) is 173 mm Hg at 25 °C. What is the vapor pressure of a solution consisting of 268 g of chloroform and 0.129 mol of a nonvolatile nonelectrolyte?

Answer 1

Given:Vapor pressure of solvent chloroform (CHCl3)p0=173mmHg

Mass of chloroform =268g

Number of moles of non-volatile non-electrolyte solute (n2) =0.129mol

The non-volatile non-electrolyte doesn't contribute to the total vapor pressure of the solution. Thus, the vapor pressure of the solution will be the vapor pressure due to solvent in the solution only i. e.

Vapor pressure of the solution=Vapor pressure of the solvent in the solution

According to Raoult's law, the vapor pressure of any component at a given temperature is equal to the mole fraction of that component in the solution multiplied by the vapor pressure of that component in the pure state.

Thus, vapor pressure of solvent = mole fraction of solvent*vapor pressure of solvent

ps =x1*p0  ​​​​​​... (1)

Here,ps= vapor pressure of solution

x1= mole fraction of the solvent

po =vapor pressure of solvent

For calculating ps​​, first calculate mole fraction of solvent

x1= n1/n1+n2

Here, n1= number of moles of solvent

n2= number of moles of solute

n1= mass of solvent /molar mass of solvent (chloroform) =268g/119.52g/mol=2.24mol

Molar mass of CHCl3=1* atomic mass of C+1*atomic mass of H+3*atomic mass of Cl

Molar mass =1*12.01g/mol+1*1.01g/mol+3*35.5g/mol

=12.01g/mol+1.01g/mol+106.5g/mol=119.52g/mol

Number of moles of solute =0.129mol

x1= 2.24mol/2.24mol+0.129mol=2.24mol/2.369mol

=0.945546644 (unit mol cancelled out)

After round off, x1=0.95

Putting the value of x1 and p0 in equation 1,we get

ps =0.95*173mmHg =164.35mmHg