1. Measuring Solubility Chapter 11

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

3. Solubility  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

4. 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.

5. Example  An 80 g sample of NaNO 3 is added to 200g of H 2 O at 20°C. Use the solubility curve in Figure 11.1 to calculate how much more NaNO 3 needs to be added to make the solution saturated with NaNO 3 at 20°C.

6. Crystallisation  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.

7. Supersaturation  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.

8. 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.

9. 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.

10. Pressure  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.

11. Your Turn  Look at worked example 11.1d on page 210.  Page 211  Question 1  Question 3  Question 5  Question 7

12. 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.

13. 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.

14.  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 Solutions

15.  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 Solutions

16.  For chemists, the most commonly used units for concentration are: Mass of solute per litre of solution Mass of solute per litre of solution Amount, in mol, of solute per litre of solution. (does this one look familiar) Amount, in mol, of solute per litre of solution. (does this one look familiar) Concentration of Solutions

17. 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.

18. 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? Solution: Concentration = mass of sulfate ions (mg) volume of mineral water (L) Concentration = 4.0 mg 0.250 L Remember 250 ml is 0.250 L 250/1000 Concentration = 16 mg/L or 16 mg L -1

19. Other units  Other units commonly used to measure volume are the cubic centimetre (cm 3 ), the cubic decimetre (dm 3 ) and the cubic metre (m 3 ).  Where 1 mL = 1cm 3, 1 L = dm 3 and 1 KL = 1 m 3

20. Your Turn  Page 215  Question 9

21. 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.

22. 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.

23. Molarity (M)  1.0 L of a 1.0 M solution of ethanol contains 1 mol of C 2 H 5 OH  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 NH 3.  Each of these solutions contains 1 mole of the solutes dissolved in solution.

24. The Equation  The amount fo solute is linked to the concentration (molarity) and volume of the solution by the relationship: n = c x V Amount, mol Concentration, mol L -1 or M Volume, L

25. Equation n = cV Or V = n c c = n V

26. 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?

27. 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. grams litre moles litre ÷ M x M

28. Worked examples  Lets do the worked examples together.  Page 213

29. Your Turn  Page 215  Question 10, 11 and 12

30. Dilution  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.

31. Dilution  Because the amount (number of moles) of solute does not change during dilution, a useful mathematical relationship exists.  Amount of solute before dilution is n 1. n 1 = c 1 V 1.  Amount of solute after dilution is n 2. n 2 = c 2 V 2.  But n 1 = n 2.  So:  c 1 V 1 = c 2 V 2.

32. Dilution c 1 V 1 = c 2 V 2  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.

33. Your Turn  Page 215  Question 13