3. Two liquids, A and B, form an ideal solution. Liquid A has a molar mass of 95

Question

3. Two liquids, A and B, form an ideal solution. Liquid A has a molar mass of 95.1 g mol-1 and liquid B a molar mass of 64.3 g mol-1. At 25 oC, the vapour pressure of A is 92.8 torr and that of B is 77.2 torr. For a mixture containing 50 g of each liquid, calculate

(i) the mol fractions of A and B in the liquid,

(ii) the partial pressures of A and B,

(iii) the total pressure,

(iv) the mol fractions of A and B in the gas phase.

(v) Comment on the differences between your answers to (i) and (iv).

3. Two liquids, A and B, form an ideal solution. Liquid A has a molar mass of 95.1 g mol and liquid B a molar mass of 64.3 g mol. At 25 °C, the vapour pressure of A is 92.8 torr and that of B is 77.2 torr. For a mixture containing 50 g of each liquid, calculate (i) the mol fractions of A and B in the liquid, (ii) the partial pressures of A and B, (ii) the total pressure, (iv) the mol fractions of A and B in the gas phase Comment on the differences between your answers to (i) and (iv)

Explanation / Answer

Answer –

We are given, mass of A = 50 g , mass of B = 50 g

Molar mass of A= 95.1 g mol-1

Molar mass of B= 64.3 g mol-1

vapour pressure of A = 92.8 torr and for B = 77.2 torr.

(i) the mol fractions of A and B in the liquid

We need to calculate the moles of each first

Moles of A = 50 g / 95.1 g mol-1

                    = 0.526 moles

Moles of B = 50 g / 64.3 g mol-1

                    = 0.778 moles

So total moles = 0.526 +0.778

                        = 1.30 moles

Mole fraction of A = moles of A / total moles

                               = 0.526 moles / 1.30 moles

                               = 0.403

Mole fraction of B = moles of B / total moles

                               = 0.778 moles / 1.30 moles

                               = 0.597

(ii) the partial pressures of A and B,

Partial pressure of A = 92.8 torr *0.403 = 37.4 torr

Partial pressure of B = 77.2 torr * 0.597 = 46.1 torr

iii) total pressure= 37.4 + 46.1

                            = 83.5 torr

(iv) the mol fractions of A and B in the gas phase.

We know formula

Mole fraction = partial pressure / total pressure

For A = 37.4 torr / 83.5 torr

             = 0.447

For B = 46.1 torr / 83.5 torr

          = 0.552

(iv) the mol fractions of A and B in the gas phase.

The mole fraction in the liquid phase for A is less than gas phase and the mole fraction in the liquid phase for B is more than gas phase.

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