Chemistry Class 12 NCERT Solutions: Chapter 2 Solutions Part 9

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Q: 16. Heptane and octane form an ideal solution. At , the vapour pressures of the two liquid components are and respectively. What will be the vapour pressure of a mixture of of heptane and of octane?

Vapour pressure of heptane

Vapour pressure of octane

We know that,

Molar mass of heptane

∴ Number of moles of heptane

Molar mass of octane

∴ Number of moles of octane

Mole fraction of heptane,

And, mole fraction of octane,

Now, partial pressure of heptane,

Partial pressure of octane,

Hence, vapour pressure of solution,


Q: 17. The vapour pressure of water is . Calculate vapour pressure of molal solution of a nonvolatile solute in it.


molal solution means 1 mol of the solute is present in of the solvent (water). Molar mass of water

Number of moles present in of water

Therefore, mole fraction of the solute in the solution is

It is given that Vapour pressure of water,

Applying the relation,

= (approximately)

Hence, the vapour pressure of the solution is .

Q: 18. Calculate the mass of a non-volatile solute (molar mass ) which should be dissolved in 114 g octane to reduce its vapour pressure to .

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,

Mass of octane,

Molar mass of octane,

Applying the relation,

Hence, the required mass of the solute is 8 g.