FreezingPtDep

FreezingPtDep - CHEM 122L General Chemistry Laboratory...

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CHEM 122L General Chemistry Laboratory Revision 1.3 The Freezing Point Depression of t -Butanol To learn about the Freezing Point Depression of Solvents. To learn about Colligative Properties. To learn about the determination of Molecular Weights. In this laboratory exercise we will measure the Freezing Point Depression of t -Butanol when a solute is added to it. We will use this data to determine the Molecular Weight of the solute. This will then be compared to the solute’s known Molecular Weight to establish the reliability of this methodology for Molecular Weight determinations. The physical properties of any solvent are inherently altered by the presence of a solute. For instance, the Boiling Point, Freezing Point, Density and Viscosity of Water, 100.0 o C, 0.0 o C, 0.9982 20 g/mL and 1.002 20 mPa sec respectively, will all change upon the addition of solutes. As an example, the addition of Ethylene Glycol (CH 2 OHCH 2 OH) to Water changes these properties relative to the amount (wt %) added: wt% Freezing Pt. [ o C] Density [g/mL] Viscosity [mPa sec] 0.0 0.00 0.99821 1.002 0.5 -0.15 0.9988 1.010 1.0 -0.30 0.9995 1.020 2.0 -0.61 1.0007 1.048 3.0 -0.92 1.0019 1.074 4.0 -1.24 1.0032 1.099 5.0 -1.58 1.0044 1.125 6.0 -1.91 1.0057 1.153 CRC Handbook of Chemistry and Physics Focusing on the Freezing Point, we note the freezing point of the solution drops as the concentration of Ethylene Glycol is increased. This is referred to as the Freezing Point Depression of the solvent. This is why Ethylene Glycol works as an Antifreeze in your car’s cooling system; it depresses the freezing point of Water so the coolant does not freeze during the winter months. The Freezing Point Depression is defined as: Δ T f = T f o - T f (Eq. 1)
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P a g e | 2 where T f is the freezing point of the solution and T f o is the freezing point of the pure solvent. If we convert the Concentration Scale from wt% to molality, the above data presents itself as: molality [mol/kg] Δ T f [ o C] 0.081 0.15 0.163 0.30 0.329 0.61 0.498 0.92 0.671 1.24 0.848 1.58 1.028 1.91 The conversion of concentration units is straight forward. For example, consider the conversion of 6 wt% to molality Ethylene Glycol in Water. First assume we have 100g of solution. At 6%, the solution is composed of 6g Ethylene Glycol and 94g of Water. Converting the mass Ethylene Glycol to number of moles, we have: # mole Eth. Gly. = 6 g / (62.07 g/mole) = 0.09667 mole Eth. Gly. Converting the mass Water to kilograms, we have: # kg Water = 94 g x (1 kg / 1000 g) = 0.094 kg So, the molality of the solution is: molality = = = 1.028 m If we plot the Freezing Point Depression data in this guise, we find a linear relationship between FP Depression and molality exists.
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P a g e | 3 In other words, the Freezing Point Depression is proportional to the solution’s molality: Δ T f ~ m (Eq. 2) The proportionality constant (slope of the line) is called the Freezing Point Depression Constant, K f . Thus, (Eq. 2) becomes:
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This note was uploaded on 12/09/2011 for the course CHEM 1 taught by Professor Staff during the Summer '11 term at Simon Fraser.

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FreezingPtDep - CHEM 122L General Chemistry Laboratory...

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