Measuring solubility

Saturated and Unsaturated• 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 that contains precisely the maximum mass of solute that can be dissolved in a particular volume of solvent at a particular temperature is called a saturated solution

• Unsaturated solution – a solution that contains less that the maximum mass of solute.

• Supersaturated solution – a solution that contains more than the maximum amount of solute.

• Solubility is measured in grams per 100 grams of water (g/100g)

• Most salts are soluble in water to some extent

• Solubility is influenced by the temperature of the solution

• If a substance does not dissolve in water it is insoluble

Solubility curve

• Used to represent the solubility of a particular substance at a given temperature.

• The graph can be used to measure the amount of solute present in a solution of a particular temperature

• Any point on the curve indicates a saturated solution.

• Any point below the curve indicates an unsaturated solution.

• Any point above the curve indicates a supersaturated solution.

Questions - Solubility Curves• What is the solubility of

KNO3 at 80C?• Which substance has the

highest solubility at 100C?• At what temperature is

NaNO3 solubility 100g/100g of water?

• At 40C, what would be the solubility of a Super, saturated and unsaturated solution of NaCl?

Crystallisation• When something cools the solute may no

longer stay dissolved in the solvent and will come out as crystals

• This is called crystallisation

• Kidney stones are formed when minerals in urine come out of the solution and form crystals.

Calculations• An 80g sample of

NaNO3 is added to 200g of H2O at 20 C. Use the solubility curve to calculate how much more NaNO3 needs to be added to the solution saturated with NaNO3 at 20C

• Pg 207 solubility curve

Solution

• 85g of NaNO3 at 20C to 100g to make saturated

• 85 X 2 = 170g of NaNO3 in 200g of water

• 170 (needed – 80(already added) = 90g need to be added

Calculations• What happens in a solution containing 50g of

KNO3 in 100g of water that is allowed to cool from 40C to 20C? Use pg 207 S curve

• At 40C normally solubility would be 70g/100g of water BUT we have 50g/100g of water

• At 20C solubililty is 35g/100g of water

• 50 – 35 = 15g will crystallise out of solution

Homework

• Pg 211

• Q 1,2,3,4,5,6,7

Solubility rules• All compounds of the ammonium ion

(NH4+), and of Alkali metal (Group IA) cations, are soluble.

• All nitrates and acetates (ethanoates) are soluble.

• All chlorides, bromides and iodides are soluble EXCEPT those of silver, lead and mercury(I).

• All sulphates are soluble EXCEPT those of silver, lead, mercury(I), barium, strontium and calcium.

Solubility rules• All carbonates, sulfites and phosphates are

insoluble EXCEPT those of ammonium and Alkali metal (Group IA) cations.

• All hydroxides are insoluble EXCEPT those of ammonium, barium and alkali metal (Group I) cations.

• All sulfides are insoluble EXCEPT those of ammonium, Alkali metal (Group I) cations and Alkali earth metal (Group II) cations.

• All oxides are insoluble EXCEPT those of calcium, barium and Alkali metal (Group I) cations; these soluble ones actually react with the water to form hydroxides (hydrolyse).

Solubility of Gases

• Gases are much less soluble in water than solid substances.

• It depends of the temperature of the liquid and the pressure of the gas.

• The higher the temperature the lower the solubility of the gas.

• Solubility increases with increasing pressure

Concentration• Concentration is the amount of solute

dissolved in a given solvent. • High concentration – high ratio of

solute:solvent. This is called a concentrated solution

• Low concentration – low ratio of solute:solvent • A dilute solution is one which has a low

concentration

Concentration

• Concentration is also defined as the amount of solute per volume of solvent

• A concentrated solution of hydrochloric acid contains 37g in a 100g solution.

• A dilute solution of hydrochloric acid contains 10g of HCl in 100g solution.

Unit Conversion

microlitre millilitre Litre kiloLitre

uL mL L kL

/1000

x1000x1000

/1000/1000

x1000

Measuring ConcentrationConcentration can be measured in several ways

1. Concentration = mass of solute in grams (g)

volume of solution in litres (L)

Concentration = g/L or gL-1

Measuring Concentration2. Another measure of concentration is

molarity or molar concentration

Molarity is defined as the number moles of solute particles per litre of solution

The symbol for molarity is M 1M is One Molar.

One Litre of a 1M solution of ethanol contains 1 mole of ethanol

Molar Concentration (molL-1) or Molarity(M)

• The number of moles per Litre of a solution is known as the molar concentration. 

• There is a relationship between mol and volume and molarity. Using units can you guess?

Concentration (molL-1) = mol/ volume in (L)

C= n

V

n = cV

 

Calculations• Calculate the molar concentration of a

solution that contains 0.105mol of KNO3 dissolved in 200mL of solution

1. Convert 200mL to L = 200/1000 = 0.2L

2. 0.105mol = 0.525 mol/L or M

0.2L

Calculations• Calculate the amount, in moles, of

ammonia(NH3) in 25.0mL of 0.3277M (mol/L)

1.Convert 25 /1000 = 0.025L

2.0.3277 mol/L x 0.025L = 0.00819 mol

Dilution• The process of adding more solvent to a

solution is called dilution

• You are not changing the amount of solute but the amount of solvent

• Therefore the amount (grams) of solute stays the same

• When a substance is diluted the solute particles have more space between them

Formula

• When we dilute something we can calculate either the new concentration or volume. This results in a formula called the dilution formula.

C1V1 = C2V2

The same amount of solute in each flask but they have different concentrations and volumes

Calculations• The concentration of a seaweed extract in a bottle of

fertiliser is 9.0g/L. When used to fertilise plans, the seaweed fertiliser must be diluted, if 10mL of seaweed is diluted to fill 2L what is the new concentration

• 10mL /1000= 0.01L • C1 = 9.0g/L V1 = 0.010L• C2? V2 = 2L• 9.0g/L x 0.01L = c2 x 2L• 9.0g/L x 0.01L = c2 = 0.045g/L

2L

Calculations• How much water must be added to 30mL of 2.5 mol/L

solution of NaOH to dilute it to 1 mol/L• The number of moles of solute does not change dilution

C1 = 2.5 mol/L v1 = 30mL c2 = 1 mol/L v2 = ?

C1v1 = c2v2

V2 = c1v1

c2

2.5mol/L x 0.030L = 0.075L

1 mol/L

75mL – 30mL = 45mL is added

Homework

• Pg 215 Q 9 - 13