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Question
Calculate the depression in the freezing point of water when 10 g of CH3CH2CHClCOOH is added to 250 g of water. Ka = 1.4 × 10−3, Kf = 1.86 K kg mol−1.
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Solution
Molar mass of CH3CH2CHClCOOH = 122.5 g mol−1
∴ No. of moles present in 10 g of CH3CH2CHClCOOH = `10/122.5` mol
= 8.16 × 10−2 mol
Molality of the solution (m) = `(8.16 xx 10^-2)/250 xx 1000` = 0.3264
Let α be the degree of dissociation of CH3CH2CHClCOOH.
CH3CH2CHClCOOH undergoes dissociation according to the following equation:
| \[\ce{CH3CH2CHClCOOH -> CH3CH2CHClCOO- + H+}\] | |
| Initial conc. | (C mol L−1) 0 0 |
| At equilibrium | C(1 − α) Cα Cα |
∴ Kα = `(C alpha * C alpha)/(C(1 - α))`
≃ Cα2
or, α = `sqrt (K_α//C)`
= `sqrt ((1.4 xx 10^-3)/0.3264)`
= 0.065
To calculate the van’t Hoff factor:
| \[\ce{CH3CH2CHClCOOH -> CH3CH2CHClCOO- + H+}\] | |
| Initial conc. | 1 0 0 |
| At equilibrium | (1 − α) α α |
Total moles = 1 + α
i = 1 + 0.065
∴ i = 1.065
ΔTf = i Kf m
= 1.065 × 1.86 × 0.3264
= 0.649
≈ 0.65°C
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| Henna is investigating the melting point of different salt solutions. She makes a salt solution using 10 mL of water with a known mass of NaCl salt. She puts the salt solution into a freezer and leaves it to freeze. She takes the frozen salt solution out of the freezer and measures the temperature when the frozen salt solution melts. She repeats each experiment. |
| S.No | Mass of the salt used in g |
Melting point in °C | |
| Readings Set 1 | Reading Set 2 | ||
| 1 | 0.3 | -1.9 | -1.9 |
| 2 | 0.4 | -2.5 | -2.6 |
| 3 | 0.5 | -3.0 | -5.5 |
| 4 | 0.6 | -3.8 | -3.8 |
| 5 | 0.8 | -5.1 | -5.0 |
| 6 | 1.0 | -6.4 | -6.3 |
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- Why did Henna collect two sets of results?
- In place of NaCl, if Henna had used glucose, what would have been the melting point of the solution with 0.6 g glucose in it?
OR
What is the predicted melting point if 1.2 g of salt is added to 10 mL of water? Justify your answer.
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The depression in the freezing point of water observed for the same amount of acetic acid, trichloroacetic acid and trifluoroacetic acid increases in the order given above. Explain briefly.
The depression in the freezing point of water observed for the same amount of acetic acid, trichloroacetic acid, and trifluoroacetic acid increases in the order given above. Explain briefly.
The depression in the freezing point of water observed for the same amount of acetic acid, trichloroacetic acid and trifluoroacetic acid increases in the order given above. Explain briefly.
The depression in freezing point of water observed for the same amount of acetic acid, trichloroacetic acid and trifluoroacetic acid increases in the order given above. Explain briefly.
The depression in freezing point of water observed for the same amount of acetic acid, trichloroacetic acid and trifluoroacetic acid increases in the order given above. Explain briefly.
The depression in freezing point of water observed for the same amount of acetic acid, trichloroacetic acid and trifluoroacetic acid increases in the order given above. Explain briefly.
