Calculate the mass of a non-volatile solute (molar mass 40 g mol−1) which should be dissolved in 114 g octane to reduce its vapour pressure to 80%.
Let the vapour pressure of pure octane be
Then, the vapour pressure of the octane after dissolving the non-volatile solute is
Molar mass of solute, M2 = 40 g mol−1
Mass of octane, w1 = 114 g
Molar mass of octane, (C8H18), M1 = 8 × 12 + 18 × 1
= 114 g mol−1
Applying the relation,
Hence, the required mass of the solute is 8 g.