28 - 1AP Chemistry Lab Molecular Weight by Freezing Point...

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1 AP Chemistry Lab Brockport High School NY USA Molecular Weight by Freezing Point Depression Mr Keefer Introduction The melting point is the temperature at which the crystal structure of a solid breaks down with increasing entropy (+ΔS). Solute particles introduced into the solvent increases the entropy, and therefore decreases the temperature of the solution in order to reach the melting or freezing point of the solution. Hence the freezing point is depressed as solute is added. This depression is the difference (ΔT f ) between the freezing point of the solvent and solute, and the freezing point of the pure solvent. A property which depends only on the number of particles present, and not on chemical composition is called a colligative property . Therefore, freezing point depression is such a property. Molality (m) is used to express the concentration of a solution in terms of moles of solute per kilogram solvent. Moles of a substance is the grams per molecular weight. The freezing point depression of a solution can be expressed as ΔT f = K f m where K f is the molal freezing point depression constant of the solvent. In this lab, we will use naphthalene as the solvent where
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28 - 1AP Chemistry Lab Molecular Weight by Freezing Point...

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