K b and K f depend only on the SOLVENT. Below are some common values. Use these

Question

Kb and Kf depend only on the SOLVENT. Below are some common values. Use these values for the calculations that follow.

0.512

-1.86

1.22

-1.99

3.67

2.53

-5.12

2.02

*Please note that T as defined above will be a negative number for freezing point depression. Therefore, Kf must also be given as a negative number.

Problem:

The boiling point of diethyl ether CH3CH2OCH2CH3 is 34.50°C at 1 atmosphere. A nonvolatile, nonelectrolyte that dissolves indiethyl ether is chlorophyll.

If 13.13 grams of chlorophyll, C55H72MgN4O5 (893.5 g/mol), are dissolved in 263.8 grams of diethyl ether.

The molality of the solution is__________ m.

The boiling point of the solution is _____________ °C.

T = T(solution) - T(pure solvent) * m = (# moles solute / Kg solvent) Kb = boiling point elevation constant. Kf = freezing point depression constant.

Explanation / Answer

The boiling point of diethyl ether CH3CH2OCH2CH3 is 34.50°C at 1 atmosphere. A nonvolatile, nonelectrolyte that dissolves indiethyl ether is chlorophyll.
If 13.13 grams of chlorophyll, C55H72MgN4O5 (893.5 g/mol), are dissolved in 263.8 grams of diethyl ether.

Molality = mol of solute / kg solvent

mol of solute = 13.13/893.5 = 0.0146950

kg of solvent = 263.8g --> 0.2638 kg

molality = mol of solute / kg solvent = 0.0146950/0.2638 = 0.0557050796 molal

a) m = 0.0557050796 molal

For BP

dTb = Kb*m*i

since nonvolatile

i = 1

For diethyl ether:

Kb = 2.02 C°/molal

then

dTb = Kb*m*i = 2.02*0.0557050796 *1 = 0.11252

Tb-mix = TB°solvent + dTb = 34.5 + 0.11252 = 34.61252

Tb-mix = 34.61252 °C

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