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Measuring Solubility. Chapter 11. Solubility. The solubility of a substance refers to the maximum amount of that substance that can be dissolved in a given quantity of solvent at a certain temperature.

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  • The solubility of a substance refers to the maximum amount of that substance that can be dissolved in a given quantity of solvent at a certain temperature.
  • A solution in which no more solute can be dissolved at that temperature is described as a saturated solution.
  • One way of measuring solubility is to determine the maximum mass of solute that can be dissolved in 100 grams of solvent at a particular temperature.
  • Solubility values allow us to compare the extent to which different substances dissolved.
  • Look at table 11.1 on page 207 and the worked example on the same page
solubility curves
Solubility Curves
  • The relationship between solubility and temperature can be represented by a solubility curve.
  • Each point in the solubility curve represents a saturated solution.
  • Any point below a curve represents an unsaturated solution for that solute.
  • An 80 g sample of NaNO3 is added to 200g of H2O at 20°C. Use the solubility curve in Figure 11.1 to calculate how much more NaNO3 needs to be added to make the solution saturated with NaNO3 at 20°C.
  • You might have noticed that honey often crystallises if you keep it in the refrigerator. The sugar becomes less soluble as the honey cools.
  • The sugar that will no longer stay dissolved comes out of solution as crystals.
  • This process is known as crystallisation.
  • With some substances it is possible to produce an unstable solution that contains more dissolved solute than in a saturated solution.
  • Such a solution is said to be supersaturated.
  • Any point above a solubility curve represents a supersaturated solution.
solubility of gases
Solubility of gases
  • Gases such as oxygen an carbon dioxide are much less soluble in water than solutes such as NaCl and sugars. Why?
  • But oxygen and carbon dioxide are present in our oceans and waterways.
  • The solubility of a particular gas in a liquid depends on the temperature of the liquid and the pressure of the gas.
temperature and gas solubility
Temperature and Gas Solubility
  • Unlike most solids, gases become less soluble as the temperature increases.
  • When you heat water, small bubbles of air form and escape the water.
  • Soft drinks contain carbon dioxide.
  • It is forced into the cans under high pressure to increase the amount that can be dissolved. When the can or bottle is opened the carbon dioxide can escape.
  • This is how drinks get flat after a certain time. As more and more carbon dioxide escapes.
your turn
Your Turn
  • Look at worked example 11.1d on page 210.
  • Page 211
  • Question 1
  • Question 3
  • Question 5
  • Question 7
concentration of solutions
Concentration of solutions
  • Before we begin I am just warning you that this is the return of the mole.
  • The mole will continue right through til the end of unit 4.
  • It is vital you understand the mole, if you are unsure of anything stop me and ask.
  • If you are unsure chances are someone else in the class is unsure too.
concentration of solutions13
Concentration of Solutions
  • When talking concentrate think of cordial.
  • If I pour the same amount of cordial into two glasses but have different amounts of water their concentrations are the same even if their volumes are different.
  • Volume and concentration are two different things.
concentration of solutions14
Concentration of Solutions
  • The concentration of a solution describes the relative amounts of solute and solvent present.
  • A solution in which the ratio of solute to solvent is high is said to be concentrated.
  • A solution in which the ratio of solute to solvent is low is said to be dilute.
concentration of solutions15
Concentration of Solutions
  • Chemists use different measures of concentration depending on the particular situation.
  • Earlier, units of grams of solute per 100 grams of solvent were used to describe the concentration of a saturated solution.
  • Other ways of expressing concentration describe the amount of solute in a given amount of solution.
  • They vary only in units used to measure the amount of solute and the amount of solutions.
concentration of solutions16
Concentration of Solutions
  • For chemists, the most commonly used units for concentration are:
    • Mass of solute per litre of solution
    • Amount, in mol, of solute per litre of solution. (does this one look familiar)
mass of solute per litre of solution
Mass of solute per litre of solution
  • This unit expressed concentration in terms of the mass of solute present in 1L of solution.
  • It is important you know how to convert metric units of volume.

mass of sulfate ions (mg)

Concentration =

volume of mineral water (L)

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



250 ml is 0.250 L


4.0 mg

Concentration =

0.250 L

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

other units
Other units
  • Other units commonly used to measure volume are the cubic centimetre (cm3), the cubic decimetre (dm3) and the cubic metre (m3).
  • Where 1 mL = 1cm3, 1 L = dm3 and

1 KL = 1 m3

your turn20
Your Turn
  • Page 215
  • Question 9
amount in mol of solute per litre of solution
Amount, in mol, of solute per litre of solution
  • Expressing concentration in moles per litre of solution allows chemists to compare relative numbers of atoms, molecules or ions present in a given volume of solution.
  • The measure of concentration, known as molarity or molar concentration, is an important one for chemists.
molarity m
Molarity (M)
  • Molarity is defined as the number of moles of solute particles per litre of solution.
  • A one molar (1 M) solution contains one mole of solute dissolved in each litre of solution.
  • A concentration of such a solution is said to be one mole per litre, 1 mol L-1 or 1M.
  • We use the term molarity to mean ‘concentration measure in moles per litre.
molarity m23
Molarity (M)
  • 1.0 L of a 1.0 M solution of ethanol contains 1 mol of C2H5OH
  • 1.0 L of a 1.0 M solution of sodium chloride contains 1 mol of NaCl
  • 2.0 L or a 0.5 mol solution of sodium chloride contains 1 mol of NaCl
  • 0.25 L of a 4.0 M solutions of ammonia contains 1 mol of NH3.
  • Each of these solutions contains 1 mole of the solutes dissolved in solution.
the equation

Amount, mol

Concentration, mol L-1 or M

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

n = c x V

Volume, L


n = cV




c =

V =



unit converstion
Unit converstion
  • The concentration units discussed here are g L-1 and mol L-1.
  • We must be able to convert from one unit to the other at times.
  • Since litres is common to both we are really just converting from grams to mole and vice versa.
  • How do we convert from grams to mole again?
don t forget molar mass
Don’t forget molar mass
  • Both molarity and molar mass use M at times. Molarity uses it as units where as molar mass uses it as a symbol. Always look carefully to determine which one M means in each question.

÷ M





x M

worked examples
Worked examples
  • Lets do the worked examples together.
  • Page 213
your turn29
Your Turn
  • Page 215
  • Question 10, 11 and 12
  • If I don’t like my cordial strong I add more water.
  • I am in effect adding more solvent (water) to a solution (cordial).
  • This process is known as dilution.
  • When a solute is diluted there is still the same amount of solute. The solute particles, however, are more widely spaced.
  • Because the amount (number of moles) of solute does not change during dilution, a useful mathematical relationship exists.
  • Amount of solute before dilution is n1.

n1 = c1V1.

  • Amount of solute after dilution is n2.

n2 = c2V2.

  • But n1 = n2.
  • So:
  • c1V1 = c2V2.

c1V1 = c2V2

  • This is called the dilution formula.
  • Since the amount of solute remains unchanged during dilution, so does the mass of the solute.

Page 214 Worked examples.

your turn33
Your Turn
  • Page 215
  • Question 13