Measuring solubility of solids

• 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.

Solubility guide for Common Ionic compounds

General Rule Exceptions

Almost all compounds that contain any of these ions are soluble:

Na+, K+, NH4+, NO3

-, CH3COO-,

Cl-, Br-, I- SO4

2-

Almost all compounds that contact any of these ions are insoluble:

S2-, CO32-, PO4

3-,

OH-

Very rare

ions are combined with Ag-

when combined with Ag+, Pb2+,Ca2+ or Ba2+

When combines with Na+, K+, NH4+

When combines with Na+, K+, NH4+,

Ca2+ or Ba2+

• Solubility can be measured by determining the maximum mass (g) of solute that can dissolve in 100 grams of solvent at a particular temperature

Examples: A maximum of 25 g of solute can be dissolved in 100g of water at 20°C – this means the solubility of solute at 20°C is 25g per 100g.

Solubility curves show relationship between solubility and temperature can be represented by a solubility curve

Measuring solubility of solids

Classification of aqueous solution:

• Aqueous solutions may be classified as saturated, unsaturated or supersaturated, depending on their relative amounts of solute and solvent.

• Solution in which no more solute can dissolve at a given temperature is described as a saturated solution.

• An unsaturated solution contains less than this maximum mass of solute, while a supersaturated solution is an unstable solution that contains more than this maximum mass of solute.

• Agitation of a supersaturated solution will cause some solute to

crystallise from solution.

Solubility of ionic salt and gases vs. temperature

• Solubility of ionic crystals increase with temperature. The increased temperature causes an increase in the vibrational motion of the ions in the crystal lattice, which destabilises the ionic bonds. Further, the increased kinetic energy of the water molecules increases the energy with which they hit the surface of the crystal and make it more likely that they will succeed in breaking the ions away from the crystal lattice.

• Solubility of gases in water will decrease with increasing temperature. As the temperature increases, the molecules gain greater kinetic energy. If they gain sufficient energy, they will become gaseous and bubble out of the solution.

Solubility Curves

• Relationship between solubility and temperature can be represented by a solubility curve

• Each point on the curve represents a saturated solution at a particular temperature.

• Any point below a curve represents an unsaturated solution for a solute

Example using solubility curve

What mass of Potassium Nitrate KNO3 will dissolve in 100g of water at 60°C?

25g of Potassium nitrate KNO3 is dissolved in 100g of water at 30°C is this solution saturated?

• . • Worked example 1

A saturated solution of sodium nitrate contains 50 g of sodium nitrate in 40 g of

water at 60°C. What is the solubility of sodium nitrate at this temperature?

• Worked example 2

A solution of potassium chlorate contains 12 g of salt in 200 g of water at 20°C.

Is the solution saturated?

• Worked example 3

A saturated solution of sodium nitrate containing 50 g of water at 80°C is

cooled to 20°C. What mass of sodium nitrate would be expected to crystallise

in the beaker?

• Solution• From the solubility graph shown in fi gure

12.1.2, the solubility of potassium chlorate at 20°C is 8 g/100 g. The solution given contains 6 g/100 g, and is therefore unsaturated.

• Solution

• At 80°C the solubility of sodium nitrate is 148 g/100 g. In 50 g of water there will therefore be 74 g of dissolved sodium nitrate. At 20°C the solubility of sodium nitrate is 88 g/100 g. In 50 g of water there will therefore be 44 g of dissolved sodium nitrate.

• Therefore 74 – 44 = 30 g of sodium nitrate will crystallise from the solution at 20°C.

• Solution

Use the solubility curves in fi gure 12.1.2 to answer the following questions.a What is the solubility of NaNO3 at 50°C?b What volume of water is needed at 30°C to make a saturated solutionfrom 45 g of NaNO3?

a If 80 mL of a saturated solution of KI at 60°C is cooled to 20°C, whatmass of KI crystallises out of the solution?b What mass of water is needed to dissolve 200 g of potassium iodideat 30°C?

a Label each of the solutions A to D as saturated, supersaturatedor unsaturated.b Describe what would happen if:i solution D was cooled to 60°Cii the side of the beaker containing solution B was tapped.