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Heat of Formation

On dissolving...

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On dissolving 0.5 g of a non-volatile non-ionic solute in 39 g of benzene, its vapour pressure decreases from 650 mm of Hg to 640 mm of Hg. The depression in freezing point of benzene (in K) upon addition of the solute is: ( Given: The molar mass and the molal freezing point depression constant for benzene are $78 g m o l^{- 1}$ and $5.12 K K g m o l^{- 1}$ , respectively

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Solution

$\frac{p_{0} - p_{s}}{p_{s}} = i (\frac{n_{s o l u t e}}{n_{s o l v e n t}}) \frac{650 - 640}{640} = 1 \times \frac{0.5 \times 78}{M \times 39} \Rightarrow M_{s o l u t e} = 64 g Δ T_{f} = K_{f} \times m o l a l i t y = 5.12 \times \frac{0.5 \times 1000}{64 \times 39} Δ T_{f} = 1.02 K$

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Q. On dissolving 0.5 g of a non-volatile non-ionic solute in 39 g of benzene, its vapour pressure decreases from 650 mm of Hg to 640 mm of Hg. The depression in freezing point of benzene (in K) upon addition of the solute is: ( Given: The molar mass and the molal freezing point depression constant for benzene are

78 g m o l^{- 1}

5.12 K K g m o l^{- 1}

Q. A very small amount of a non volatile solute(that does not dissociate) is dissolved in

56.8

cc of benzene (density

0.889

g/cc). At room temperature the vapour pressure of this solution is

98.88

mm Hg while that of benzene is

100

mm Hg. Find the molality of this solution freezing temperature of this solution is

0.73

degree lower than that of benzene, hat is the value of molal freezing point depression constant of benzene.

Q. 1 g of non-electrolyte solute dissolved in 50g of benzene lowered the freezing point of benzene by 0.40K. The freezing point depression constant of benzene is 5.12 K kg/mol. Find the molar mass of the solute.

Q. The molar volume of liquid benzene (density

= 0.877 g m L^{- 1}

) increases by a factor of

2750

as it vaporises at

20^{\circ} C

while in equilibrium with liquid benzene. At

27^{\circ} C

when a non-volatile solute (that does not dissociate) is dissolved in

54.6 c m^{3}

of the benzene vapour pressure of this solution, is found to be

98.88 m m H g

Calculate the freezing point of the solution.
Given: Enthalpy of vaporisation of benzene

(l) = 394.57 J / g

Molal depression constant for benzene

= 5.12 K k g . m o l^{- 1}

Freezing point of benzene

= 278.5 K

Q. 1.00 g of a non-electrolyte solute (molar mass 250g mol

^{- 1}

) was dissolved in 51.2 g of benzene.If the freezing point depression constant K

_{f}

of benzene is 5.12 K kg mol

^{1}

, the freezing point of benzene will be lowered by:

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