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EXPERIMENT 2 COLLIGATIVE PROPERTIES: MOLECULAR WEIGHT DETERMINATION Objectives 1. To practice freezing point determination of liquids. 2. To determine the molecular weight of an unknown by freezing point depression. INTRODUCTION A solution is a homogeneous mixture (has a constant composition throughout) which consists of a dissolved substance, the solute, and a dissolving medium, the solvent. A solute need not to be a solid. It can be a gas, such as carbon dioxide, CO2, or a liquid, such as ethylene glycol (antifreeze in car's cooling system). If the solution contains two liquids, the liquid that is in the greater amount is called the solvent. The most common solvent is water. The concentration of solutions can be described quantitatively in many ways, such as molarity, molality, and mole fraction. Some properties of a solution depend on the concentration of particles, such as molecules or ions in the solution, rather than on particular characteristics of the molecular or ionic substance. These properties are called colligative properties. The number of particles in a solvent can affect the freezing or boiling point of solvent. To express the effect of concentration on freezing point or boiling point, molality is used in place of molarity. Molality (m) is the number of moles of solute per 1 kilogram (or 1000 g) of solvent. For example, a two molal (2 m) solution contains two moles of solute dissolved in 1 kilogram of solvent. When a solute is dissolved in a solvent, the vapor pressure of the solvent is reduced. The amount of the reduction depends upon the number of solute particles in a given amount of solvent. Since both the freezing point and the boiling point of a liquid depend on its vapor pressure, introduction of a solute in a solvent will change the solvent's freezing and boiling points. The freezing point of the liquid is lowered (freezing point depression or lowering) by the addition of a solute. In contrast, the addition of a solute increases the boiling point of a liquid. The change in freezing point and boiling point is directly proportional to the concentration of the solute particles. One mole or 6.02 x 10^23 particles of a molecular solute (non-ionic) dissolved in 1000 grams of water (making 1 molal solution) lowers the freezing point of water by 1.86 °C. This temperature interval (1.86 °C) is the molal freezing point depression constant for water which is expressed as Kf = 1.86 °C/m. If the solute is an ionic compound total number of ions released per formula of the solute will determine the "effective molality". For example, one mole of the ionic solute, NaCl, contains 2 moles of solute particles (1 mole of Na+ and 1 mole of Cl- ions). The effective molality of a solution prepared by

EXPERIMENT 2

COLLIGATIVE PROPERTIES: MOLECULAR WEIGHT DETERMINATION

Objectives

1. To practice freezing point determination of liquids.

2. To determine the molecular weight of an unknown by freezing point depression.

INTRODUCTION

A solution is a homogeneous mixture (has a constant composition throughout) which consists of a dissolved substance, the solute, and a dissolving medium, the solvent.

A solute need not to be a solid. It can be a gas, such as carbon dioxide, CO2, or a liquid, such as ethylene glycol (antifreeze in car's cooling system). If the solution contains two liquids, the liquid that is in the greater amount is called the solvent. The most common solvent is water.

The concentration of solutions can be described quantitatively in many ways, such as molarity, molality, and mole fraction. Some properties of a solution depend on the concentration of particles, such as molecules or ions in the solution, rather than on particular characteristics of the molecular or ionic substance. These properties are called colligative properties. The number of particles in a solvent can affect the freezing or boiling point of solvent. To express the effect of concentration on freezing point or boiling point, molality is used in place of molarity. Molality (m) is the number of moles of solute per 1 kilogram (or 1000 g) of solvent. For example, a two molal (2 m) solution contains two moles of solute dissolved in 1 kilogram of solvent.

When a solute is dissolved in a solvent, the vapor pressure of the solvent is reduced. The amount of the reduction depends upon the number of solute particles in a given amount of solvent. Since both the freezing point and the boiling point of a liquid depend on its vapor pressure, introduction of a solute in a solvent will change the solvent's freezing and boiling points. The freezing point of the liquid is lowered (freezing point depression or lowering) by the addition of a solute. In contrast, the addition of a solute increases the boiling point of a liquid. The change in freezing point and boiling point is directly proportional to the concentration of the solute particles.

One mole or 6.02 x 10^23 particles of a molecular solute (non-ionic) dissolved in 1000 grams of water (making 1 molal solution) lowers the freezing point of water by 1.86 °C. This temperature interval (1.86 °C) is the molal freezing point depression constant for water which is expressed as Kf = 1.86 °C/m. If the solute is an ionic compound total number of ions released per formula of the solute will determine the "effective molality". For example, one mole of the ionic solute, NaCl, contains 2 moles of solute particles (1 mole of Na+ and 1 mole of Cl- ions). The effective molality of a solution prepared by

$EXPERIMENT 2 COLLIGATIVE PROPERTIES: MOLECULAR WEIGHT DETERMINATION Objectives 1. To practice freezing point determination of liquids. 2. To determine the molecular weight of an unknown by freezing point depression. INTRODUCTION A solution is a homogeneous mixture (has a constant composition throughout) which consists of a dissolved substance, the solute, and a dissolving medium, the solvent. A solute need not to be a solid. It can be a gas, such as carbon dioxide, CO2, or a liquid, such as ethylene glycol (antifreeze in car's cooling system). If the solution contains two liquids, the liquid that is in the greater amount is called the solvent. The most common solvent is water. The concentration of solutions can be described quantitatively in many ways, such as molarity, molality, and mole fraction. Some properties of a solution depend on the concentration of particles, such as molecules or ions in the solution, rather than on particular characteristics of the molecular or ionic substance. These properties are called colligative properties. The number of particles in a solvent can affect the freezing or boiling point of solvent. To express the effect of concentration on freezing point or boiling point, molality is used in place of molarity. Molality (m) is the number of moles of solute per 1 kilogram (or 1000 g) of solvent. For example, a two molal (2 m) solution contains two moles of solute dissolved in 1 kilogram of solvent. When a solute is dissolved in a solvent, the vapor pressure of the solvent is reduced. The amount of the reduction depends upon the number of solute particles in a given amount of solvent. Since both the freezing point and the boiling point of a liquid depend on its vapor pressure, introduction of a solute in a solvent will change the solvent's freezing and boiling points. The freezing point of the liquid is lowered (freezing point depression or lowering) by the addition of a solute. In contrast, the addition of a solute increases the boiling point of a liquid. The change in freezing point and boiling point is directly proportional to the concentration of the solute particles. One mole or 6.02 x 10^23 particles of a molecular solute (non-ionic) dissolved in 1000 grams of water (making 1 molal solution) lowers the freezing point of water by 1.86 °C. This temperature interval (1.86 °C) is the molal freezing point depression constant for water which is expressed as Kf = 1.86 °C/m. If the solute is an ionic compound total number of ions released per formula of the solute will determine the "effective molality". For example, one mole of the ionic solute, NaCl, contains 2 moles of solute particles (1 mole of Na+ and 1 mole of Cl- ions). The effective molality of a solution prepared by$

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