The vapour pressure of pure benzene at 353 K is 10.0268 x 104 Nm-2 and...
Given data:
Vapour pressure of pure benzene at 353 K, P₁ = 10.0268 × 10⁴ Nm⁻²
Vapour pressure of solution, P₂ = 9.8960 × 10⁴ Nm⁻²
Density of benzene, d = 8.149 × 10² g dm⁻³
To find: Osmotic pressure of the solution
Formula used:
For a dilute solution, the osmotic pressure (π) is given by the equation:
π = (n/V)×RT
where n is the number of moles of the solute, V is the volume of the solution, R is the gas constant and T is the temperature.
The number of moles of the solute can be calculated as:
n = (m/M)
where m is the mass of the solute and M is its molar mass.
Solution:
Molar mass of benzene, C₆H₆ = 6 × 12.01 + 6 × 1.008 = 78.11 g mol⁻¹
Let us assume that we have 1000 g of the solution.
Then, the mass of benzene in the solution = (1000 - m) g
From Raoult's law, we know that:
P₂ = X₂ × P₁
where X₂ is the mole fraction of benzene in the solution.
Mole fraction of benzene in the solution can be calculated as:
X₂ = moles of benzene / total moles
Let the moles of benzene be n₂.
Then, the total moles can be calculated as:
n₁ + n₂ = (mass of benzene / Molar mass of benzene) + (mass of solute / Molar mass of solute)
Since the solute is non-volatile, its vapour pressure is negligible and hence its mole fraction can be assumed to be equal to 1.
Therefore, the above equation can be simplified as:
n₂ = (mass of benzene / Molar mass of benzene)
Thus, we have:
X₂ = n₂ / (n₁ + n₂)
X₂ = (mass of benzene / Molar mass of benzene) / [(mass of benzene / Molar mass of benzene) + (mass of solute / Molar mass of solute)]
Substituting the given values, we get:
9.8960 × 10⁴ = [(1000 - m) / 78.11] / [m / (M × 78.11)] × 10.0268 × 10⁴
Solving for m, we get:
m = 39.24 g
Mass of solute, w = (1000 - m) g = 960.76 g
Number of moles of the solute, n = (w / M) = (960.76 / M) mol
Volume of the solution, V = (w / d) = (960.76 / 8.149) dm³ = 117.8 dm³
Substituting the given values in the equation for osmotic pressure, we get:
π = (n/V)×RT
π = [(960.76 / M) / 117.8] × 8