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07 - Determining Molar Mass by Freezing Point Depression

07 - Determining Molar Mass by Freezing Point Depression -...

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Determining Molar Mass by Freezing Point Depression Purpose Compare freezing points of a pure solvent and a known solution of the solvent to determine the change in freezing point, T f Calculate the molal freezing point depression constant (K f ) for the solvent. Determine the molar mass of an unknown solute using data from the proceeding procedures. Introduction A solution is a homogeneous mixture of substances, comprised of at least two components, the major component being the solvent and the minor the solute. The solvent is the component that defines the phase of a solution; it is the material into which the solute is introduced. The solute is the substance that dissolves in a solvent to form a solution. A solution can be made from any combination of the phases of matter. In this lab, two solids are mixed together, then melted to complete the solution process. There are a number of ways of expressing concentration quantitatively, but the ones commonly used in AP Chemistry are molarity (M), molality (m), mole fraction ( χ ) and mass percent. M = mol solute / L solution m = mol solute / kg solvent χ = mol solute / mol total mass % = (mass solute / mass total) * 100% Since volume varies as temperature varies, any concentration measures that includes volume in its definition is temperature-dependent. Therefore a stated molarity, for instance, must include the temperature at which the solution was measured. Ratios that compare only moles or mass are not temperature-dependent, since those quantities do not vary as temperature does. Colligative properties are a set of characteristics that differ for a pure solvent and a solution of the solvent. The presence of particles of solute in the solvent interferes with the normal behavior of the solvent, especially the characteristic of vapor pressure. It is not important what the chemical

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07 - Determining Molar Mass by Freezing Point Depression -...

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