Solutions and Colligative Properties

Chemistry – Dr. May Notes Solutions and Colligative Properties

What is a solution?

A solution is a homogeneous mixture (solid, liquid, or gas) having any substance that is evenly dispersed throughout another substance.

A solution may appear to be a pure substance but is a mixture.

The substance that is the dissolving medium is the solvent. The solute is the material that is dissolved.

The Process of Solvation.

The polar water molecule can surround positive and negative ions. These ions are said to be hydrated. As the crystals of an ionic material are converted into hydrated ions, the process of dissociation occurs.

Dissolving a solute in a solvent is called solvation. Like dissolves like means that polar solvents dissolve polar solutes.

Molecular solvation occurs when the polar solvent molecules attract the polar solute molecules.

When there is no limit to the solubility of one substance in another (up to 100%), the materials are said to be miscible. Water and ethanol are examples of miscible materials.

Materials that do not dissolve in each other, like oil and water, are said to be immiscible.

Solubility

When a solvent can dissolve no more solute, the solution is said to be saturated. If more solute can be dissolved in the solvent, the solution is said to be unsaturated.

Solubility does not have a precise meaning but as a rule means that a 0.1 Molar solution can be prepared at 0oC.

Solubility is the amount of material needed to make a saturated solution at a given temperature. It is usually expressed as grams of solute per 100 grams of solvent at 0oC (or some other temperature like 24oC)

1 Factors Affecting Solubility

Solubility increases with an increase in the temperature of the solution. When a solution is saturated at elevated temperatures then cooled, crystals usually form. The solution will hold less solute at cool temperatures. If crystals do not form, the solution is said to be supersaturated.

Pressure will increase the solubility of gas in solvent. Carbon dioxide in soda is an example. The gas escapes when pressure is decreased (the soda is opened).

Ionic Equations and Precipitation Reactions

Reactions in Solution.

AgNO3 + KCl  AgCl  + KNO3

In the above reaction silver nitrate and potassium chloride are soluble in water. When they react in aqueous solution, potassium nitrate and silver chloride are formed. Silver chloride is insoluble in water and forms a precipitate. The solution contains the following ions:

+ +   Ag K NO3 Cl

The only compounds possible are the reactants and the products. When the silver chloride is formed, the insoluble precipitate removes Ag+ and Cl from the mixture.

Writing Net Ionic Equations

+  AgNO3  Ag + NO3 KCl  K+ + Cl

+  +  +  Ag + NO3 + K + Cl  AgCl  + K + NO3

+  The above equation is called an ionic equation. K and NO3 are called spectator ions since they remain ions in solution. The net ionic equation is determined by canceling out the ions that occur on both sides of the equation.

+  +  +  Ag + NO3 + K + Cl  AgCl  + K + NO3

Ag+ + Cl  AgCl 

2 Colligative Properties of Solutions

Boiling Point Elevation and Freezing Point Depression

Solutions have higher boiling points and lower freezing points than the pure solvents. This is why adding salt (sodium chloride or calcium chloride) to snow and ice causing melting. The freezing point is lowered.

These are called colligative properties. The ionization of salts also makes water conduct electricity. Salts are called electrolytes.

The number of solute particles, not their size or whether they are ions or molecules, determine how the solute will affect the boiling and freezing points of water.

Molality

Molality (m) is the number of moles of solute per kilogram of solvent.

Molarity (M) can be thought of as weight per volume while Molality (m) is weight per weight.

To determine how many grams of barium chloride (BaCl2) you would need to prepare a 0.40 m solution in 100 grams of water, first determine the molecular weight of barium chloride: 208.3 g/mole.

Then we know that molality (m) = moles of BaCl2  kg of water.

0.40 = moles  0.100 kg moles of BaCl2 = 0.04 grams = moles x MW grams = 0.04 x 208.3 = 8.33 grams

Calculations Involving Colligative Properties

The formula to calculate the boiling point elevation of water is:

Tb = kb x m

o Tb is the increase in the boiling point in C kb is a constant with the value of 0.51 m is molality of the solution in moles/kg of solvent

Tf = kf x m

o Tf is the difference in freezing point between solution and pure solvent in C kf is a constant with the value of 1.86 m is molality of the solution in moles/kg of solvent

3 Solvents other than water have different kb and kf values. The calculations work the same regardless of solvent and solute.

Calculating Molar Mass

By using the above colligative relationships combined with the fact that molar mass (molecular weight)(MW) is related to grams and moles, we can determine the molecular weight of an unknown material by knowing the freezing point depression (Tb) caused by a given amount of solute in a given amount of water.

36.0 grams of material are dissolved in 100 grams of water. The solution begins to freeze at 3.72oC. What is the molar mass of the solute?

Tf = kf x m m ‗  Tf ‗ 3.72 ‗ 2.0 moles/kg solvent kf 1.86

moles of material ‗ 2.0 ‗ 0.20 moles 0.1 (100 grams)

MW ‗ grams ‗ 36.0 ‗ 180 grams/mole (molar mass) (MW) moles 0.20