Introduction to Solution Chemistry Concentrations of Solutions

Packet #23

Introduction I

Recall that solutions are homogenous mixtures that may be solid, liquid or gases.

Recall that a solution has two parts

Solute

Solvent

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Introduction II Properties of Solutions

A sinkhole forms when the roof of a cave weakens from being dissolved by groundwater and suddenly collapses. One recorded sinkhole swallowed a house, several other buildings, five cars, and a swimming pool…

Introduction III Solution Formation

The composition of the solvent and the solute determines whether a substance will dissolve.

Sugar in water

Salt in water

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Introduction IV Solution Formation

Solution Formation

What factors determine the rate at which a substance dissolves?

1. Stirring (agitation)

2. Temperature

3. The surface area of the dissolving particles

Solutions

Stirring Temperature Surface Area

Three Factors

Introduction V Solution Formation

A cube of sugar in cold tea dissolves slowly.

Little surface area

Cold temperature

No agitation

Introduction VI Solution Formation

Granulated sugar dissolves in cold water more quickly than a sugar cube, especially with stirring.

More surface area

More agitation

Introduction VII Solution Formation

Granulated sugar dissolves very quickly in hot tea.

More surface area

Increase temperature

Introduction VIII Solution Formation

Anything that increases solute to solvent particle contact increases the rate of dissolving.

Stirring, heating, pulverizing

Solubility

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Introduction Solubility

Solubility is a chemical property referring to the ability for a given substance, the solute, to dissolve in a solvent.

The quantity of solute needed to just saturate {not to be confused with saturated molecule} a given amount of solvent.

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How is solubility usually expressed?

The solubility of a substance is the maximum amount of solute that can dissolve in a given quantity of a solvent at a specified temperature and pressure.

Solubility is often expressed in grams of solute per 100 g of solvent.

Introduction II Solubility

Solubility Curve

Solubility Curve II

A solubility curve traces the solubility of a substance with increasing temperature

Each point on the curve represents the composition of a saturated solution (not to be confused with saturated molecule) at a different temperature.

Solubility Solubility Curve III

A saturated solution contains the maximum amount of solute for a given quantity of solvent at a given temperature and pressure.

The point when the solvent will accept no more solute

Any additional solute would drop to the bottom of the vessel

Points on the curve

Solubility Dynamic Equilibrium

In a saturated solution, the rate of dissolving equals the rate of crystallization, so the total amount of dissolved solute remains constant.

This is a dynamic equilibrium.

Solubility Solubility Curve IV

An unsaturated solution contains less solute than a saturated solution at a given temperature and pressure.

A solution that has not reached its saturation point.

Points below the curve

Factors Affecting Solubility

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Factors Affecting Solubility I

The mineral deposits around hot springs result from the cooling of the hot, saturated solution of minerals emerging from the spring.

Mineral deposits form around the edges of this hot spring because the hot water is saturated with minerals.

As the water cools, some of the minerals crystallize because they are less soluble at the lower temperature.

What conditions determine the amount of solute that will dissolve in a given solvent?

Temperature affects the solubility of solids, liquids, and gases

Pressure also affects the solubility of gases (not liquids or solids)

Factors Affecting Solubility II

Factors Affecting Solubility III

Increasing temperature:

Increases the solubility of most solids

Decreases the solubility of gases

Factors Affecting Solubility IV

Changing pressure

Changes in pressure have little effect on the solubility of solids and liquids, but pressure strongly influences the solubility of gases.

Gas solubility increases as the vapor pressure of the gas above the solution increases.

Open bottles of soda go flat…

Factors Affecting Solubility V

A supersaturated solution is unstable and contains more solute than it can theoretically hold at a given temperature.

The crystallization of a supersaturated solution can be initiated if a very small crystal, called a seed crystal, of the solute is added.

Factors Affecting Solubility VI

A supersaturated solution is clear before a seed crystal is added.

Factors Affecting Solubility VII

Crystals begin to form in the solution immediately after the addition of a seed crystal.

Factors Affecting Solubility VIII

Excess solute crystallizes rapidly.

Concentration

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Concentration I

When scientists prepare solutions, they are usually interested in strength or concentration.

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Concentrations of Solutions II

Water must be tested continually to ensure that the concentrations of contaminants do not exceed established limits. These contaminants

include metals, pesticides, bacteria, and even the by-products of water treatment.

How do you determine the concentration of a solution? The concentration of a solution is a measure of the

amount of solute that is dissolved in a given quantity of solvent. A dilute solution is one that contains a small amount of

solute.

A concentrated solution contains a large amount of solute.

Concentration III

Parts per Million

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Parts per Million (ppm) is often used to express the concentrations of very dilute solutions.

This is useful when very little solute is dissolved in a very large volume of water; for example the amount of dissolved oxygen in a lake.

Parts per Million of Solute = (Mass of Solute/Mass of solution ) * 106

ppm = (mass solute/mass solution) X 1,000,000

Parts per Million {PPM} I

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A certain gas has a concentration in water of 2 milligrams per 100 grams of water. What is the concentration of the gas in parts per million?

Parts per Million II Example #1

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A 1.0 L sample of water from a stream contains 0.008g of dissolved oxygen. What is the concentration in parts per million?

Parts Per Million III Example #2

Molarity

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Molarity I

Molarity is defined as the number of moles of solute dissolved in 1 liter of solution.

It is the ratio between the number of moles of solute and number of liters of solution.

M = (n/V)

n = moles of solute

V = volume of solution (L)

M = mol/L

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Molarity (M) is the unit of concentration for solutions.

To calculate the molarity of a solution, divide the moles of solute by the volume (in liters) of the solution.

Reference Table T

Molarity II

Molarity Making Molar Solutions I

To make a 0.5-molar (0.5M) solution, first add 0.5 mol of solute to a 1-L volumetric flask half filled with distilled water.

Molarity IV Making Molar Solutions II

Swirl the flask carefully to dissolve the solute.

Molarity V Making Molar Solutions III

Fill the flask with water exactly to the 1-L mark.

How many moles of a solute is needed to prepare 2.5 liters of a 0.60 M solution?

Molarity VI Example #1

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Molarity VII Example #2

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Molarity VIII Example #3

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Molarity IX Example #4

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When a solute is dissolved in a solvent, the particles of the solute bond with the particles of the solvent.

Causes vapor pressure to be lower than that of pure solvent.

Boiling points raise

Freezing points lower

These are colligative properties

They depend on the number of solute particles present and the nature of the solvent

Colligative properties do not depend on the actual identity of the solute.

Molarity X Why Important?

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Collogative Properties

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A property that depends ONLY upon the number of solute particles, and not upon their identity, is called a colligative property.

Colligative Properties I

What are three colligative properties of solutions?

Vapor-pressure lowering

Boiling-point elevation

Freezing-point depression

Colligative Properties II

Dissolving a solute in a solvent will

Lower the vapor pressure

Raise the boiling point and

Lower the freezing point

The more you dissolve the greater the change.

Colligative Properties III

Vapor-Pressure Lowering

In a pure solvent, equilibrium is established between the liquid and the vapor.

Vapor-Pressure Lowering II

In a solution, solute particles reduce the number of free solvent particles able to escape the liquid.

Equilibrium is established at a lower vapor pressure.

Vapor-Pressure Lowering III

Three moles of glucose dissolved in water produce 3 mol of particles because glucose does not dissociate.

Vapor-Pressure Lowering

Three moles of sodium chloride dissolved in water produce 6 mol of particles because each formula unit of NaCl dissociates into two ions.

This solution will have a lower vapor pressure than the previous one.

Vapor-Pressure Lowering

Three moles of calcium chloride dissolved in water produce 9 mol of particles because each formula unit of CaCl2 dissociates into three ions.

This solution will have the lowest vapor pressure.

16.3

Which solution has the lowest vapor pressure at STP?

a. 1 M KCl

b. 1 M CH3OH

c. 1 M MgBr2

Vapor-Pressure Lowering

Freezing-Point Depression

The freezing-point depression of aqueous solutions makes walks and driveways safer when people sprinkle salt on icy surfaces to make ice melt.

The melted ice forms a solution with a lower freezing point than that of pure water.

16.3

Dissolving a solute in water will lower its freezing point.

The difference in temperature between the freezing point of a solution and the freezing point of the pure solvent is the freezing-point depression. The more concentrated the solution the lower the freezing

point.

Freezing-Point Depression

16.3

Freezing-Point Depression

Which solution would have the lowest freezing point?

a. 1 mole of MgBr2 in 1 kg water

b. 1 mole of CH3OH in 1 kg water

c. 1 mole NaNO3 in 1 kg water

Dissolving a solute in water will raise its boiling point.

The difference in temperature between the boiling point of a solution and the boiling point of the pure solvent is the boiling-point elevation. The more concentrated the solution the higher the boiling

point.

Boiling-Point Elevation

Boiling-Point Elevation

Which solution would have the highest boiling point?

a. 1 mole of NaCl in 1 kg water

b. 1 mole of NaCl in 2 kg water

c. 1 mole of NaCl in 3 kg water

Molality

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Molality

There are situations when the amount of solvent in a solution must be known.

When scientists examine how a solute affects the properties of the solvent.

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Molality

The number of moles of solute dissolved in 1 kilogram of solvent.

The ratio between the number of moles of solute and the mass of solvent (kg).

m = n/kg

m = molaity

N = number of moles of solute

kg = to kilograms of solvent.

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Sucrose (table sugar) has a molar mass of 342 grams per mole. What is the molality of a solution prepared by dissolving 34.2 grams of sucrose in 200 grams of water?

Molality Example #1

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Dilutions

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Making Dilutions

What effect does dilution have on the total moles of solute in a solution?

Nothing. Diluting a solution reduces the number of moles of solute per unit volume, but the total number of moles of solute in solution does not change.

Making Dilutions

Making a Dilute Solution

Making Dilutions II

Making Dilutions III

Volume-Measuring Devices

Since the total number of moles of solute remains unchanged upon dilution, you can use this equation:

M1V1 = M2V2

Where M1 and V1 are the molarity and volume of the initial solution, and M2 and V2 are the molarity and volume of the diluted solution.

Making Dilutions IV

How do you prepare 100 ml of 0.40M MgSO4 from a stock solution of 2.0M MgSO4?

Making Dilutions V Example #1

Making Dilutions VI Example #2

Review

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