Measuring Solubility

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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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## Measuring Solubility

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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.
• A solution in which no more solute can be dissolved at that temperature is described as a saturated solution.
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
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.
Example
• 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.
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.
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.
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
• Unlike most solids, gases become less soluble as the temperature increases.
• When you heat water, small bubbles of air form and escape the water.
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.
• Look at worked example 11.1d on page 210.
• Page 211
• Question 1
• Question 3
• Question 5
• Question 7
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 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 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 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 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
• 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?

Solution:

Remember

250 ml is 0.250 L

250/1000

4.0 mg

Concentration =

0.250 L

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

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

• Page 215
• Question 9
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 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 (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.

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

Equation

n = cV

Or

n

n

c =

V =

V

c

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
• 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

grams

moles

litre

litre

x M

Worked examples
• Lets do the worked examples together.
• Page 213
• Page 215
• Question 10, 11 and 12
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.
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 n1.

n1 = c1V1.

• Amount of solute after dilution is n2.

n2 = c2V2.

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

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.