Measuring SolubilityMeasuring Solubility

Chapter 11Chapter 11

SolubilitySolubility

The The solubilitysolubility of a substance refers to the of a substance refers to the maximum amount of that substance that maximum amount of that substance that can be dissolved in a given quantity of can be dissolved in a given quantity of solvent at a certain temperature.solvent at a certain temperature.

A solution in which no more solute can be A solution in which no more solute can be dissolved at that temperature is described dissolved at that temperature is described as a as a saturated solutionsaturated solution..

SolubilitySolubility

One way of measuring solubility is to determine One way of measuring solubility is to determine the maximum mass of solute that can be the maximum mass of solute that can be dissolved in 100 grams of solvent at a particular dissolved in 100 grams of solvent at a particular temperature.temperature.

Solubility values allow us to compare the extent Solubility values allow us to compare the extent to which different substances dissolved. to which different substances dissolved.

Look at table 11.1 on page 207 and the worked Look at table 11.1 on page 207 and the worked example on the same pageexample on the same page

Solubility CurvesSolubility Curves

The relationship between The relationship between solubility and temperature solubility and temperature can be represented by a can be represented by a solubility curve. solubility curve.

Each point in the Each point in the solubility curve solubility curve represents a saturated represents a saturated solution.solution.

Any point below a curve Any point below a curve represents an represents an unsaturated solution for unsaturated solution for that solute.that solute.

ExampleExample

An 80 g sample of NaNOAn 80 g sample of NaNO33 is added to is added to

200g of H200g of H22O at 20O at 20°C. Use the solubility °C. Use the solubility

curve in Figure 11.1 to calculate how curve in Figure 11.1 to calculate how much more NaNOmuch more NaNO33 needs to be added to needs to be added to

make the solution saturated with NaNOmake the solution saturated with NaNO33 at at

2020°C. °C.

CrystallisationCrystallisation

You might have noticed that honey often You might have noticed that honey often crystallises if you keep it in the refrigerator. crystallises if you keep it in the refrigerator. The sugar becomes less soluble as the The sugar becomes less soluble as the honey cools.honey cools.

The sugar that will no longer stay The sugar that will no longer stay dissolved comes out of solution as dissolved comes out of solution as crystals. crystals.

This process is known as This process is known as crystallisationcrystallisation..

SupersaturationSupersaturation

With some substances it is possible to With some substances it is possible to produce an unstable solution that contains produce an unstable solution that contains more dissolved solute than in a saturated more dissolved solute than in a saturated solution. solution.

Such a solution is said to be Such a solution is said to be supersaturatedsupersaturated..

Any point above a solubility curve Any point above a solubility curve represents a supersaturated solution.represents a supersaturated solution.

Solubility of gasesSolubility of gases

Gases such as oxygen an carbon dioxide Gases such as oxygen an carbon dioxide are much less soluble in water than are much less soluble in water than solutes such as NaCl and sugars. Why?solutes such as NaCl and sugars. Why?

But oxygen and carbon dioxide are But oxygen and carbon dioxide are present in our oceans and waterways. present in our oceans and waterways.

The solubility of a particular gas in a liquid The solubility of a particular gas in a liquid depends on the temperature of the liquid depends on the temperature of the liquid and the pressure of the gas. and the pressure of the gas.

Temperature and Gas SolubilityTemperature and Gas Solubility

Unlike most solids, gases become less Unlike most solids, gases become less soluble as the temperature increases.soluble as the temperature increases.

When you heat water, small bubbles of air When you heat water, small bubbles of air form and escape the water.form and escape the water.

PressurePressure

Soft drinks contain carbon dioxide.Soft drinks contain carbon dioxide. It is forced into the cans under high It is forced into the cans under high

pressure to increase the amount that can pressure to increase the amount that can be dissolved. When the can or bottle is be dissolved. When the can or bottle is opened the carbon dioxide can escape. opened the carbon dioxide can escape.

This is how drinks get flat after a certain This is how drinks get flat after a certain time. As more and more carbon dioxide time. As more and more carbon dioxide escapes. escapes.

Your TurnYour Turn

Look at worked example 11.1d on page Look at worked example 11.1d on page 210.210.

Page 211Page 211 Question 1Question 1 Question 3Question 3 Question 5Question 5 Question 7Question 7

Concentration of solutionsConcentration of solutions

Before we begin I am just warning you that Before we begin I am just warning you that this is the return of the mole.this is the return of the mole.

The mole will continue right through til the The mole will continue right through til the end of unit 4. end of unit 4.

It is vital you understand the mole, if you It is vital you understand the mole, if you are unsure of anything stop me and ask.are unsure of anything stop me and ask.

If you are unsure chances are someone If you are unsure chances are someone else in the class is unsure too.else in the class is unsure too.

Concentration of SolutionsConcentration of Solutions

When talking concentrate think of cordial.When talking concentrate think of cordial. If I pour the same amount of cordial into two glasses but If I pour the same amount of cordial into two glasses but

have different amounts of water their concentrations are have different amounts of water their concentrations are the same even if their volumes are different. the same even if their volumes are different.

Volume and concentration are two different things. Volume and concentration are two different things.

The concentration of a solution describes The concentration of a solution describes the relative amounts of solute and solvent the relative amounts of solute and solvent present. present.

A solution in which the ratio of solute to A solution in which the ratio of solute to solvent is high is said to be solvent is high is said to be concentratedconcentrated..

A solution in which the ratio of solute to A solution in which the ratio of solute to solvent is low is said to be solvent is low is said to be dilutedilute..

Concentration of SolutionsConcentration of Solutions

Chemists use different measures of Chemists use different measures of concentration depending on the particular concentration depending on the particular situation. situation.

Earlier, units of grams of solute per 100 grams of Earlier, units of grams of solute per 100 grams of solvent were used to describe the concentration solvent were used to describe the concentration of a saturated solution.of a saturated solution.

Other ways of expressing concentration describe Other ways of expressing concentration describe the amount of solute in a given amount of the amount of solute in a given amount of solution.solution.

They vary only in units used to measure the They vary only in units used to measure the amount of solute and the amount of solutions. amount of solute and the amount of solutions.

Concentration of SolutionsConcentration of Solutions

For chemists, the most commonly used For chemists, the most commonly used units for concentration are:units for concentration are: Mass of solute per litre of solutionMass of solute per litre of solution Amount, in mol, of solute per litre of solution. Amount, in mol, of solute per litre of solution.

(does this one look familiar)(does this one look familiar)

Concentration of SolutionsConcentration of Solutions

Mass of solute per litre of solutionMass of solute per litre of solution

This unit expressed concentration in terms This unit expressed concentration in terms of the mass of solute present in 1L of of the mass of solute present in 1L of solution.solution.

It is important you know how to convert It is important you know how to convert metric units of volume.metric units of volume.

A 250ml glass of orange-flavoured mineral water contains A 250ml glass of orange-flavoured mineral water contains 4.0mg of sulfate ions. What is the concentration (in mg L4.0mg of sulfate ions. What is the concentration (in mg L-1-1) ) of sulfate ions in the mineral water? of sulfate ions in the mineral water?

Solution:Solution:Concentration =

mass of sulfate ions (mg)

volume of mineral water (L)

Concentration =4.0 mg

0.250 L

Remember250 ml is 0.250 L250/1000

Concentration = 16 mg/L or 16 mg L-1

Other unitsOther units

Other units commonly used to measure Other units commonly used to measure volume are the cubic centimetre (cmvolume are the cubic centimetre (cm33), the ), the cubic decimetre (dmcubic decimetre (dm33) and the cubic metre ) and the cubic metre (m(m33).).

Where 1 mL = 1cmWhere 1 mL = 1cm33, 1 L = dm, 1 L = dm33 and and 1 KL = 1 m1 KL = 1 m33

Your TurnYour Turn

Page 215Page 215 Question 9Question 9

Amount, in mol, of solute per litre of Amount, in mol, of solute per litre of solutionsolution

Expressing concentration in moles per litre Expressing concentration in moles per litre of solution allows chemists to compare of solution allows chemists to compare relative numbers of atoms, molecules or relative numbers of atoms, molecules or ions present in a given volume of solution.ions present in a given volume of solution.

The measure of concentration, known as The measure of concentration, known as molarity molarity or or molar concentrationmolar concentration, is an , is an important one for chemists.important one for chemists.

Molarity (M)Molarity (M)

Molarity is defined as the number of moles Molarity is defined as the number of moles of solute particles per litre of solution.of solute particles per litre of solution.

A one molar (1 M) solution contains one A one molar (1 M) solution contains one mole of solute dissolved in each litre of mole of solute dissolved in each litre of solution.solution.

A concentration of such a solution is said A concentration of such a solution is said to be to be one mole per litre, one mole per litre, 1 mol L1 mol L-1-1 or 1M or 1M..

We use the term molarity to mean We use the term molarity to mean ‘concentration measure in moles per litre.‘concentration measure in moles per litre.

Molarity (M)Molarity (M)

1.0 L of a 1.0 M solution of ethanol contains 1 1.0 L of a 1.0 M solution of ethanol contains 1 mol of Cmol of C22HH55OHOH

1.0 L of a 1.0 M solution of sodium chloride 1.0 L of a 1.0 M solution of sodium chloride contains 1 mol of NaClcontains 1 mol of NaCl

2.0 L or a 0.5 mol solution of sodium chloride 2.0 L or a 0.5 mol solution of sodium chloride contains 1 mol of NaClcontains 1 mol of NaCl

0.25 L of a 4.0 M solutions of ammonia contains 0.25 L of a 4.0 M solutions of ammonia contains 1 mol of NH1 mol of NH33..

Each of these solutions contains 1 mole of the Each of these solutions contains 1 mole of the solutes dissolved in solution.solutes dissolved in solution.

The EquationThe Equation

The amount fo solute is linked to the The amount fo solute is linked to the concentration (molarity) and volume of the concentration (molarity) and volume of the solution by the relationship:solution by the relationship:

n = c x VAmount, mol

Concentration, mol L-1 or M

Volume, L

EquationEquation

n = cV

Or

V =nc

c =nV

Unit converstionUnit converstion

The concentration units discussed here The concentration units discussed here are g Lare g L-1-1 and mol L and mol L-1-1..

We must be able to convert from one unit We must be able to convert from one unit to the other at times. to the other at times.

Since litres is common to both we are Since litres is common to both we are really just converting from grams to mole really just converting from grams to mole and vice versa. and vice versa.

How do we convert from grams to mole How do we convert from grams to mole again?again?

Don’t forget molar massDon’t forget molar mass

Both molarity and molar mass use M at times. Both molarity and molar mass use M at times. Molarity uses it as units where as molar mass Molarity uses it as units where as molar mass uses it as a symbol. Always look carefully to uses it as a symbol. Always look carefully to determine which one M means in each question.determine which one M means in each question.

grams

litre

moles

litre

÷ M

x M

Worked examplesWorked examples

Lets do the worked examples together. Lets do the worked examples together. Page 213Page 213

Your TurnYour Turn

Page 215Page 215 Question 10, 11 and 12Question 10, 11 and 12

DilutionDilution

If I don’t like my cordial strong I add more If I don’t like my cordial strong I add more water.water.

I am in effect adding more solvent (water) I am in effect adding more solvent (water) to a solution (cordial). to a solution (cordial).

This process is known as dilution. This process is known as dilution. When a solute is diluted there is still the When a solute is diluted there is still the

same amount of solute. The solute same amount of solute. The solute particles, however, are more widely particles, however, are more widely spaced. spaced.

DilutionDilution

Because the amount (number of moles) of solute Because the amount (number of moles) of solute does not change during dilution, a useful does not change during dilution, a useful mathematical relationship exists. mathematical relationship exists.

Amount of solute before dilution is nAmount of solute before dilution is n11..nn11 = c = c11VV11..

Amount of solute after dilution is nAmount of solute after dilution is n22..nn22 = c = c22VV22..

But nBut n11 = n = n22.. So:So: cc11VV1 1 = c= c22VV22..

DilutionDilution

cc11VV1 1 = c= c22VV22

This is called the dilution formula. This is called the dilution formula. Since the amount of solute remains Since the amount of solute remains

unchanged during dilution, so does unchanged during dilution, so does the mass of the solute.the mass of the solute.

Page 214 Worked examples.Page 214 Worked examples.

Your TurnYour Turn

Page 215Page 215 Question 13Question 13