UNIT 9 Molality & Colligative Properties

Molarity is the most widely used form of quantitative concentration, but it is not the only one. There are actually a number of other forms of quantitative concentration. Each is used to support very specific purposes. Some of these other forms include: – Parts per thousand – Parts per million – Mole fractions – Molality OTHER FORMS OF QUANTITATIVE CONCENTRATION

Molality is defined as moles of solute per kilogram of solvent One of the major reasons for having molality is that it supports shipping and transportation. These processes are mainly based on weight of material being shipped and not volume. Comparison: – Molarity = moles of solute per liter of solution – Molality = moles of solute per kilogram of solvent – Since the density of water is 1 L/kg, there is only a very minor difference if the solvent is water MOLALITY & COMPARISON TO MOLARITY

Simple enough, divide the number of moles of solute by the mass of the solvent in kilograms. Since both of this quantities can be provided in different units, conversions may be required before calculation. The most common of these are provide below: – If the solute value is given in grams, it must first be converted to moles. – If the solvent is represented in grams, it must first be converted to kilograms. MOLALITY CALCULATIONS

One area of Chemistry that uses molality is Colligative Properties Colligative properties are physical properties dependent on the number of particles present. The most prominent of these are: – Freezing point depression of a solution – Boiling point of a solution – Osmotic pressure within a solution The focus of this module will be the effects on the freezing and boiling points of solutions COLLIGATIVE PROPERTIES

The presence of particles in a solution will result in the lowering of the freezing point of a solution. The amount of depression is directly dependent on the number of particles present. The particles present can be the result of either the dispersion of covalent compound molecules or insoluble ionic compounds or the result of dissociation of soluble ionic compounds. Since soluble ionic compounds dissociate into ions, which are particles, the impact of these compounds is much greater than either insoluble compounds or molecular compounds that cannot dissociate. FREEZING POINT DEPRESSION OF A SOLUTION

The presence of particles in a solution will result in the raising of the boiling point of a solution. The amount of elevation is directly dependent on the number of particles present. The particles present can be the result of either the dispersion of covalent compound molecules or insoluble ionic compounds or the result of dissociation of soluble ionic compounds. Since soluble ionic compounds dissociate into ions, which are particles, the impact of these compounds is much greater than either insoluble compounds or molecular compounds that cannot dissociate. BOILING POINT ELEVATION OF A SOLUTION

Insoluble compounds that will not disperse and covalent compounds that cannot dissolve in the solvent (immiscible) will have no impact on freezing point depression or boiling point elevation. Since insoluble compounds cannot dissociate, one mole of solute, only if it can be dispersed (not precipitate), will provide one mole of particles. Since molecular compounds cannot dissociate, one mole of solute, only if it is miscible (able to dissolve), will provide one mole of particles. Since soluble ionic compounds will dissociate into ions, the amount of impact is dependent on the number of ions resulting from the dissociation. DEGREE OF IMPACT OF DIFFERENT TYPES OF SOLUTES

Both freezing point depression (FPD) and boiling point elevation (BPE) have very similar equations. The only difference is in the constants used. – K f = constant associated freezing (cryoscopic constant) – K b = constant associated boiling (ebullioscopic constant) Freezing point depression (FPD): – (molality)(particles per mole of solute) (K F ) Boiling Point Elevation (BPE) – (molality)(particles per mole of solute) (K b ) The constants are dependent on the solvent. The values for water are as follows: – K f = °C kg / mol – K b = °C kg / mol AMOUNT OF CHANGE CALCULATIONS