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The vapor pressure of a solution containing an unknown quantity of C2H6O2 in 100 grams of water is 14.12 mm Hg at 220C. At the same temperature, pure water has a vapor pressure of 19.83 mm Hg. How many grams of C2H6O2 are in the solution?

Answer :

Using Raoult's Law, we can calculate that 3846.43 grams of C₂H₆O₂ are in the solution.

According to Raoult's law, the vapor pressure of a component in an ideal solution is directly proportional to its mole fraction in the solution.

[tex]P_{total} = X_A \times P_A + X_B \times P_B[/tex]

where:

[tex]P_{total}[/tex] is the total vapor pressure of the solution = 14.12 mm Hg,

[tex]X_A[/tex] and [tex]X_B[/tex] are the mole fractions of components A and B (in this case, water and C₂H₆O₂, respectively), and

[tex]P_A[/tex] (19.83 mm Hg) and [tex]P_B[/tex] are the vapor pressures of components A and B in their pure states.

Let the unknown quantity of C₂H₆O₂ in the solution be "x" grams.

Calculating the mole fraction of C₂H₆O₂ ([tex]X_B[/tex]) in the solution

The mole fraction of C₂H₆O₂ ([tex]X_B[/tex]) can be calculated from the following formula:

[tex]X_B[/tex] = moles of C₂H₆O₂ / total moles in the solution

Since we are given that the solution contains 100 grams of water and "x" grams of C₂H₆O₂, the total moles in the solution can be calculated as follows:

Total moles = moles of water + moles of C₂H₆O₂

Moles of water = 100 grams / molar mass of water (18.015 g/mol)

Moles of C₂H₆O₂ = x / molar mass of C₂H₆O₂ (46.07 g/mol)

Further, rearrange and use Raoult's law to calculate "x":

[tex]X_B[/tex] = ([tex]P_{total}[/tex] - [tex]X_A[/tex] * [tex]P_A[/tex]) / [tex]P_A[/tex]

[tex]X_A[/tex] = moles of water / total moles in the solution

[tex]X_B[/tex] = ([tex]P_{total}[/tex] - [tex]X_A[/tex] * [tex]P_A[/tex]) / [tex]P_B[/tex]

Moles of water = 100 g / 18.015 g/mol ≈ 5.547 mol

Moles of Câ‚‚H₆Oâ‚‚ = x g / 46.07 g/mol ≈ x / 46.07 mol

Total moles in the solution = 5.547 mol + (x / 46.07) mol

[tex]X_A[/tex] = [tex]\frac{5.547 mol }{5.547 mol + \frac{x}{46.07 mol} }[/tex]

[tex]X_A[/tex] = 5.547 / (5.547 + 0.0215x)

[tex]X_B[/tex] = (14.12 mm Hg - [tex]X_A[/tex] * 19.83 mm Hg) / 19.83 mm Hg

Now, solve for "x":

[tex]X_B[/tex] = (14.12 - 19.83 × 5.547 / (5.547 + 0.0215x)) / 19.83

[tex]X_B[/tex] ≈ -102.56781 / (19.83 + 0.0215x)

We know that [tex]X_B[/tex] cannot be negative.

[tex]X_B[/tex] ≈ 102.56781 / (19.83 + 0.0215x)

102.56781 / (19.83 + 0.0215x) = 1 (since it is a mole fraction)

0.0215x = 82.73781

x = 82.73781 / 0.0215

x ≈ 3846.428 g

Therefore, approximately 3846.43 grams of C₂H₆O₂ are in the solution.

To learn more about the molar fraction

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Rewritten by : Jeany