Solutions and

Concentration

A solution is made up of two

or more substances

combined together.

The state of matter for a

solution depends upon the

state of the solvent.

The solvent is the compound

present most often

The solute is the compound(s)

present less often then the

solvent

The interactions between the solvent and the

solute determine the extent of solvation

Solutions

Ionic compounds

in water

When a solid ionic compound is dissolved in

water, first the compound must be

surrounded on the molecular level with

solvent.

Once surrounded, it will leave the ionic solid

and enter the solvent.

Next, the ionic compound will separate into

ions

Finally, the water molecules will separate the

two ions and surround them in a specific

orientation according to the ion charge

When the solvent is water and the solute is an

ionic compound, the dissolving process is

called hydration.

Expressing

Concentration

When planning a reaction, it is important to

know the concentration of the solution.

For only qualitative tests, simply knowing the solution is concentrated or dilute may be specific enough.

Usually this is not enough information.

For quantitative information there are five

methods to express concentration.

1. Mass percentage

2. Parts per million

3. Mole fraction

4. Molarity

5. Molality

Mass Percentage

Mass of component in solution * 100

Total mass of solution

A solution is made by dissolving 13.5 g of

glucose, C6H12O6, in 0.100 kg of water.

What is the mass percentage of solute in

this solution?

13.5 g * 100 =

113.5 g

11.9%

Parts per million (ppm)

ppm is used for very dilute solutions

Examples; air pollution, ozone levels and water contamination

Mass of component in solution *106

Total mass of solution

1 ppm = 1 g of solute for each million grams

of solution

Problem A 2.5 g sample of ground water was found to

contain 5.4g of Zn2+. What is the concentration of Zn2+ in ppm?

5.4*10-6 g *106

2.5 g

=2.2ppm

Problems What is the mass percentage of 25g of HCl in

0.250 kg of water?

Problem What is the ppm of 11g of As in 25 g of

water?

Mole fraction (X) Moles of component

Total moles of all components

X is the symbol for mole fraction.

A subscript is usually added to inform what

compound was evaluated for the mole

fraction of the solution (more then one

substance may be dissolved in a solution)

Problem What is the mole fraction of 5 moles of HCl in

11 moles of NaCl and 75 moles of water?

Molarity (M) Moles of solute

Liters of solution

Molality (m) Moles of solute

Kilograms of solvent

Notice the difference between molarity and

molality, these two measurements can be

easily be confused.

Problem

• What is the molality of 5

moles of HCl in 17 kg of

water?

Solutions and

Solubility

For any solution the process of solvation is

opposed by crystallization

In a saturated solution, the maximum amount

of solute has been dissolved.

If more is added, it will collect at the bottom.

An unsaturated solution means more solute

may be dissolved

A supersaturated solution contains more

solute then expected.

These solutions are produced by dissolving solute at a high temperature and slowly cooling it.

Adding one grain of solute will result in lots of solute crystallizing.

Factors affecting

solubility

The composition of the solute and the solvent

will determine if the substances will mix.

“like dissolves like”

• Pairs of liquids that will mix are miscible

• Example: water and alcohol

• Pairs of liquids that will not mix are immiscible

• Example: oil and water

Increase dissolving

Solids 1. Increase the temperature

2. Increase the surface area of the solute

3. Agitate the solution

Increase dissolving

Gas 1. Increase the surface area of the solvent

2.Decrease the temperature

3. Increase the pressure

Henry’s law The relationship between pressure and

solubility is shown using Henry’s law:

Cg = kPg

Cg = the solubility of the gas in the solution (usually in molarity M)

Pg = the partial pressure of the gas

K = Henry’s law constant

K changes for every solute-solvent pair