Year 11 Chapter 11:~ Measuring Solubility

11.1 Measuring Solubility

The solubility of a substance refers to the maximum amount of that substance that can be ______in a given quantity of a solvent at a certain temperature.

A solution that no more solute can be dissolved at that temperature is described as a ______solution. To measure solubility you need to determine the maximum amount of solute that can be dissolved in ______grams of solvent.

Solubility values allow you to compare the extent to which different solutes dissolve.

Example:

A maximum of 6g of solute can be dissolved in 20g of water at 20°C. What is the solubility?

Solubility Curves – The relationship between solubility and temperature can be represented by these curves (see Figure 11.1).

Example:

An 80g sample of NaNO3 is added to 200g of H2O at 20°C. Use the solubility curve in Fig 11.1 to calculate how much more NaNO3 needs to be added to make the solution saturated with NaNO3 at 20°C.


Crystallisation

If a hot saturated solution of potassium nitrate is cooled, crystals of the solute will appear. This happens because potassium nitrate also becomes less soluble as the temperature falls. The potassium nitrate crystallises from solution.

Example:

What happens if a solution containing 50g of potassium nitrate in 100g of water is allowed to cool from 40°C to 20°C?

Supersaturation

With some substances, it is possible to produce an unstable solution that contains more dissolved solute that in a saturated solution, this is a supersaturated.

A supersaturated solution of potassium nitrate can be prepared by cooling a saturated solution very carefully so that crystallisation does not occur.

Questions: 1, 2, 3, 4 & 5.

Solubility of Gases

Gases such as oxygen and carbon dioxide are much less soluble in water then solids such as sodium chloride and sucrose. Aquatic life relies on dissolved oxygen in the water and aquatic plants rely on dissolved ______.

The solubility of a particular gas in a liquid depends on the ______of the liquid and the pressure of the gas.

Temperature and gas solubility

Unlike most solids, gases become ______soluble as the temperature increase. You may have notice when you heat water small bubbles of air form and escape from the water.

Pressure and gas solubility

The solubility of gases increases with ______pressure. When you open a can or bottle of soft drink you hear the sound of the gas escaping.

Example: Using table 11.2

Calculate the percentage of oxygen that would be lost from a pond, saturated with oxygen, if the temperature of the water in the pond rose from 0°C to 20°C.

Mass of Oxygen in 1kg:

at 0°C = _____ g

at 20°C = _____ g

So, mass of oxygen lost:

Therefore % of oxygen lost = ______X 100

0.069

= ______%

QUESTIONS: 6, 7, & 15.

11.2 Concentration of Solutions

The concentration of a solution describes the relative amounts of solute and solvent present.

J  A solution in which a ratio of solute to solvent is high is said to be ______.

J  A solution in which a ratio of solute to solvent is low is said to be ______.

You have measured concentration in g/100g so far, there are other ways of expressing concentration where they measure the amount of solute in a given amount of solution.

Mass of solute per litre of solution: g/L (g L-1) or mg/L

Amount, in mol, of solute per litre of solution: mol/L (mol L-1) or M

*Example of cordial

Mass of solute per litre of Solution

This unit expresses concentration in terms of the mass solute present in 1 litre of solution (g/L)

Example: A 250mL glass of orange-flavoured mineral water contains 4.0mg of sulfate ions. What is the concentration (in mgL-1) of sulfate ions in the mineral water.
1mL of water = 1g of water

m(SO42-) = 4.0mg

m(solvent) = 250mL = 0.250L

mass of solute (mg)

Concentration = volume of solution (L)
Amount, in mol, of Solute per litre of Solution

This allows chemists to compare relative numbers of atoms, molecules or ions present in a given volume of solution. The measure of concentration is known as molarity (M) or molar concentration.

Molarity if defined as the number of moles of solute particles per ______of solution. A one molar (1M) solution contains _____ mole solute dissolved in each one ______of solution: 1 mole per litre, 1 mol L-1, 1mol/L or M.

The amount of solute is linked to the concentration (molarity) and volume of the solution:

Unit Conversion

Examples:

1. Calculate the molar concentration of a solution that contains 0.105 mol of potassium nitrate dissolved in 200mL of solution:

n = 0.105 mol

V = 200ml = _____ L

c = ?

n .

C(KNO3) = V

2. Calculate the amount, in moles, of ammonia (NH3) in 25.0mL of a 0.3277M ammonia solution.

n = ?

V = 25.0mL = ______L

c = 0.3277M

3. Calculate the concentration, in mol L-1, of a solution that contains 16.8mg of silver nitrate (AgNO3) dissolved in 150mL of solution.

Step 1 Convert:

n = ?

V = 150mL = 0.150 L

c = ?

Step 2: Calculate n using mass equation

n = ?

m = 16.8mg = 0.0168g

M = 169.9 g mol-1

Step 3: Calculate c using concentration equation

Dilution

The process of adding more solvent to a solution is known as dilution. When a solution is diluted, the solute particles are more widely spread.

Looking at figure 11.11 The concentration of the solutions has decreased by a factor of 5, however the amount of solute has not changed though they are just spaced further apart.

Amount doesn’t change therefore n does not change.


Example:

1.  The concentration of a seaweed extract in a bottle of seaweed fertiliser solution is 9.0 g L-1. When used to fertilise plants the seaweed extract must be diluted. If 10mL of seaweed fertiliser is diluted with water to fill a 2.0L container, what is the new concentration?

c1V1 =c2V2

c1 = 9.0 g/L

V1 = 10mL = 0.010L

C2 = ?

V2 = 2.0L

2.  How much water must be added to 30mL of 2.5M solution of a sodium hydroxide solution to dilute it to 1.0M.

QUESTIONS:

J  10, 11, 12ac, 13bd, E12345, 17, 19, 21, 22ac, 23ab, 24 & 26.

J  27, 28, 29, 30, 31, 32, 33 & 36.