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How do I find the freezing point depression of a Naphthalene Solution?

Feb.17, 2010 in Questions & Answers

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I added .219g in 20mL of cyclohexane. The freezing point of cyclohexane was 6.5 C, With Naphthalene it was 4.0 C, How do I use these factors to find the freezing point depression?

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Comments (2)

2 Comments on “How do I find the freezing point depression of a Naphthalene Solution?”

Steve O

email me, if this is not the case:
The freezing point of pure cyclohexane was 6.5 C
when 0.219g of Naphthalene was dissolved in 20mL of cyclohexane it froze at 4.0C
find the molal freezing point depression constant for cyclohexane

first moles, using molar mass
0.219g of Naphthalene @ 128.17 g/mol = 0.001709 moles naphthalene

now grams of cyclohexane using its density @ http://en.wikipedia.org/wiki/Cyclohexane
20 ml cyclohexane @ 0.779 g/ml = 15.58 grams of cyclohexane

now molality of the mix
(0.001709 moles naphthalene) / (0.01558 kilograms of cyclohexane) = 0.1097 molal

now the formula
dT = Kf (molality)

2.5 C = Kf (0.1097m)

Kf = 22.8 C/molal

nottoo far off, text book is 20.2
Hart

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