Determination of the Molecular Weight of an Unknown NonElectrolyte by its Effect on the
Freezing Point Depression in tButyl Alcohol
Olivia Downs
2/21/11
CHM 1046L / #85606
Megan Conkling & Chris Reyes
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The purpose of this experiment was to determine the molecular weight of the unknown
electrolyte by finding its’ freezing point depression in tButyl alcohol.
The freezing point was depressed because it is lower than the freezing point of a pure
solvent. Freezing points are caused to be depressed if the solution is high in concentration. The
molecular weight was calculated using freezing point depression through a series of equations.
First, the molality (Moles solute ÷ kg solvent) was found. In this experiment, molality can also
be expressed as: m= ΔT ÷ fkrp. To find the molecular weight of a solution, the equation is: MW=
g solute ÷ moles solute. To find the moles solute, this equation can be used: Moles solute= (ΔT)
× (kg solvent) ÷ (kf). The kf is the molal freezing point depression constant which is always:
8.37◦C/m. Before solving for the molecular weight, the grams of solute was found by taking the
volume in milliliters of the unknown added and multiplying that volume by the density of the
unknown(D=m ÷ v).
After the grams solute is calculated, it was inserted into the equation to find
molecular weight and divided by the calculated moles solute. To find the percent error of the
experiment, use the equation: Theoretical MWActual MW ÷Theoretical MW ×100. Phase
changes occurred in this experiment when each solution was placed into each different
temperature bath. The phase change that occurred was from liquid to solid when the tButyl
solution was placed into the pure bath. Dynamic equilibrium occurs in the solution as the
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 Spring '11
 Simmons
 Chemistry, Mole, Mass, Freezingpoint depression, tbutyl alcohol

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