The purpose of this lab is to determine how much the freezing point of a pure substance changes when impurities are added. We will first determine the freezing point of pure naphthalene (C8H10), then add a small amount of p-dichlorobenzene (C6H4Cl2) and determine the new freezing point.
Recall that a liquid freezes when it becomes solid; it does not necessarily have to become cold.
The relationship between amount of impurities and freezing point is
given by the equation
DT = km
where DT is the amount the freezing point lowers or “depresses”, k is a constant unique to every substance, and m is molality, defined as “moles of solute per kilogram of solvent.”
The purpose of this activity is to determine the freezing point depression constant for naphthalene, and to compare our experimental value to the accepted value.
|test tube #||__________|
|1.||mass of test tube||__________||g|
|2.||mass of test tube + naphthalene||__________||___|
|3.||freezing point of pure naphthalene||__________||___|
|5.||freezing point of naphthalene + p-dichlorobenzene||__________||___|
|7.||mass of test tube + naphthalene + p-dichlorobenzene||__________||___|
|Results (attach all calculations)|
|a.||mass of p-dichlorobenzene||__________||___|
|b.||moles of p-dichlorobenzene||__________||___|
|c.||mass of naphthalene||__________||Kg|
|d.||molality of solution||__________||___|
|e.||change in freezing point||__________||___|
|f.||k from experiment||__________||___|
[Freezing Point Depression score sheet][MHS Chem page]