Question

vapour pressure of a solution of heptane & octane is given by the equation: P(sol.)(mm Hg) = 35 + 65x, where x is the mole fraction of heptane. Calculate the vapour pressure of pure octane.

Answer 1

Explanation:

Vapour pressure of heptane p10 = 105.2 kPa

Vapour pressure of octane p20= 46.8 kPa

As we know that, Molar mass of heptane (C7H16) = 7 × 12 + 16 × 1 = 100 g mol - 1

∴ Number of moles of heptane = 26/100 mol = 0.26 mol

Molar mass of octane (C8H18) = 8 × 12 + 18 × 1 = 114 g mol - 1

∴ Number of moles of octane = 35/114 mol = 0.31 mol

Mole fraction of heptane, x1 = 0.26 / 0.26 + 0.31

= 0.456

And, mole fraction of octane, x2 = 1 - 0.456 = 0.544

Now, partial pressure of heptane, p1 = x2 p20

= 0.456 × 105.2

= 47.97 kPa

Partial pressure of octane,p2 = x2 p20

= 0.544 × 46.8 = 25.46 kPa

Hence, vapour pressure of solution, ptotal = p1 + p2

= 47.97 + 25.46

= 73.43 kPa