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305ColligativeProperties

# 305ColligativeProperties - Colligative properties Consider...

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Lesson 13 1 Lesson 13 Colligative properties Consider a solution formed by dissolving a non-volatile solute (such as sugar) in a liquid solvent (such as water). If this solution obeys both Raoult’s and Henry’s laws it is called an ideal solution . We will study the following 4 properties of ideal solutions ( colligative properties ) : Vapour-pressure lowering Boiling-point elevation Freezing-point depression Osmotic pressure The equations that will be used apply only for dilute solutions , where the assumption of ideality is valid. Lesson 13 Vapour pressure lowering Raoult’s law for a binary solid/liquid solution: P = P liquid + P solid = P 1 + P 2 = x 1 P vap, 1 + x 2 P vap, 2 If the solid is non-volatile P vap ,2 = 0: P = x 1 P vap, 1 Since x 1 < 1 , P < P vap, 1 .This vapour pressure lowering ( D P 1 ) is calculated with the equation: D P 1 = P P vap ,1 = x 1 P vap, 1 P vap, 1 = (1 x 1 ) P vap ,1 = x 2 P vap ,1 Lesson 13 Estimation of molar mass using D P 1 The vapour pressure depression can be related to the molar mass of the solute with the equation: This equation can be used to estimate the molar mass of an unknown solid dissolved in a known liquid 1 , 1 1 2 2 2 2 1 , 1 2 2 1 / / /

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305ColligativeProperties - Colligative properties Consider...

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