Solubility

Solubility is a means of comparing the extent to which different solutes can dissolve in a particular solvent at a definite temperature.

Saturated and unsaturated solutions

Saturated solution of a solute at a particular temperature is one which contains as much solute as it can dissolve at that temperature in the presence of undissolved solute particles. The composition of a saturated solution is not affected by the presence of excess solute. 

Unsaturated solution has a lower concentration of solute than a saturated solution and can dissolve more solutes, if added until it becomes saturated. 

Supersaturated solution: a supersaturated solution is one which can contain more of the solute than it can normally dissolve at that temperature. A supersaturated solution, however, is unstable and the excess solute will separate out if the solution is disturbed, or if a tiny crystal of the solute is added to it.

Determination of solubility

1. Preparing a saturated solution of the given solute at the specified temperature

2. Taking a known mass of the saturated solution

3. Heating to dryness the known mass of the saturated solution, so that the exact mass of the solvent and the solute in the saturated solution at that temperature can be calculated

*Solubility curves

Source: www.google.com 

Importance of solubility curves

1. It enables us to read solubilities at any required temperature covered by the graph

2. It enables us to determine the mass of the solute that will crystallize out when a saturated solution at a higher temperature is cooled to a lower temperature.

3. It enables us to find out the ordering which solutes will separate when a mixed solution is allowed to cool (fractional crystallization).

4. It enables us to predict the temperature at which a hot solution of known concentration will begin to crystallize out.

5. It enables pharmacists to determine the amount of solid drugs that must be dissolved in a given quantity of solvent to give a prescribed drug mixture. 

6. It enables chemists and researchers to determine the most suitable solvent to be used at varied temperatures for the extraction of essential chemicals from various natural sources.

Solubility of gases (solutions of gases in liquids)

The solubility of a gas in a liquid is usually indicated by an absorption coefficient which is defined as the volume of the gas at s.t.p which saturates 1cm³ of the liquid at a given temperature and at 1 atmospheric pressure.

Factors affecting solubility of gases

The solubility of a gas in liquid is affected by pressure, temperature and the chemical reaction of the gas with the liquid.

The relationship between pressure and solubility is expressed by Henry’s law. This states that the mass of gas dissolved by a given volume of solvent, to give a saturated solution is directly proportional to the pressure of the gas, provided that the temperature remains constant and there is no reaction between the gas and solvent.

Solubility of solid (solution of solids in liquids)

If a non volatile solute is dissolved in a liquid, the vapour pressure of the solution is lower than that of the pure solvent. This is because part of the surface of the solution is occupied by solute molecules and so the number of solvent molecules escaping is reduced while the number of those returning is unaltered.

Calculation on solubility

25.0g of potassium chloride were dissolved in 80g of distilled water at 30°C; calculate the solubility of the solute in mol dm³ (k=39 cl=35.5)

Solution

Molar mass of kcl= 74.5 

25.0g of kcl contains = 25.0 / 74.5

                                  =0.336mole

80g of water at 30°c dissolved 0.336 moles of kcl. :.1000cm³ of water at 30^°C will dissolve 1000×336/80 = 4.2moles of Kcl

At 30°C, the solubility of potassium chloride in water is 4.2mole dm-³ 

Past questions

1. In general, an increase in temperature increases the solubility of a solute in water because? (Jamb 1995)

1. More solute molecules collide with each other 
2. Most solutes dissolve with the evolution of heat
3. More solute dissolve with the evolution of heat
4. Most solutes dissolve with absorption of heat

Answer: C

2. From the diagram above, the mass of crystals deposited when 1dm³ of a saturated solution of NaCl is cooled from 80°C to 60°C is? (Jamb 1999)

1. 176.0g
2. 58.50g
3. 11.70g
4. 5.85g[Na=23, cl=35.5]

Answer: A

Solution: solubility at 80°C = 10molsdm-³

                 solubility at 60°C = 8molsdm^-3

                 :. On cooling from 80°C to 60°C

     (10-8)mol dm-3 precipitate i.e 2mol dm^-3 =(2×58.5) = 117

NaCL(23+35.5) = 58.5

:. 117+58.5= 176.0g

3. The diagram above is the solubility curve of solute, X. Find the amount of X deposited when 500cm of a solution of X is cooled from 60°C to 20°C? (Jamb 2000)

1. 0.750mole
2. 0.950mole
3. 2.375moles
4. 4.750moles

Answer: A

Solution: solubility at 60° > 5.5moldm³

                 solubility at 20 > 4.0moldm³

      :. 1.5moldm³ of X will be precipitation on cooling 1000cm³ of solution X will precipitate (^1.5/₂) = 0.750mole

4.

At what temperature are the solubilities of L and K the same? (Jamb 2001)

1. 75°C
2. 100°C
3. 90°C
4. 82°C

Answer: D

5. Defined solubility. 

Answer:

Solubility is a means of comparing the extent to which different solutes can dissolve in a particular solvent at a definite temperature.

6. Defined saturated solution.

Answer: 

Saturated solution of a solute at a particular temperature is one which contains as much solute as it can dissolve at that temperature in the presence of undissolved solute particles.

7. Defined supersaturated solution.

Answer:a supersaturated solution is one which can contain more of the solute than it can normally dissolve at that temperature

8. Defined unsaturated solution

Answer:

Unsaturated solution has a lower concentration of solute than a saturated solution and can dissolved more solutes, if added until it becomes saturated