Solubility and Concentration

ChemistryMrs. Coyle

Part I Solubility:

The ability of a solute to dissolve in a solvent.

Expressed as g solute/100g H2O

or g solute/100mL H2O

Soluble

• Soluble: a solute that has appreciable solubility.

Miscible

Substances that dissolve in each other.

Solubility Curves

Precipitate: solute that comes out of solution.•

Types of Solutions Saturated contains the maximum amount of

solute that can be dissolved at the given conditions of T and P.

Unsaturated contains less than the saturated amount of solute.

Supersaturated contains more than the saturated amount of solute by dissolving at higher temp and then cooling.

A supersaturated solution crystallizes after a seed crystal is introduced.

http://www.chem.ufl.edu/~itl/2045/change/C12F11.GIF

Rates of Solution

Does every candy you eat take the same time to dissolve?

Factors affecting the rate of dissolving:

Surface Area

Stirring

Temperature (average kinetic energy)

Factors Affecting Solubility

Temperature (affects solid, liquid and gaseous solutes)

Pressure (affects gaseous solutes)

Effect of temperature on solubility of gases.Example:

Compare the amount of oxygen dissolved in the waters of the arctic ocean to the amount of oxygen dissolved in warm tropical waters.

Effect of temperature on solubility of solids. Example:

Can you dissolve more sugar in warm water or in cold water?

Solubility Curves

Effect of Pressure on solubility of solids. Negligible.

Effect of Pressure on solubility of gases. Henry’s Law: the amount of gas dissolved in

a solution is directly proportional to the pressure of the gas above the solution.

The higher the pressure the higher the solubility of the gas.

Effect of Pressure on solubility of gases.

http://www.chem.ufl.edu/~itl/2045/lectures/lec_i.html

Example

An unopened bottle of soda has a pressure of 5atm above the liquid, so the concentration of CO2 in the soda is high.

Compare that to a pressure of 1 atm above the liquid when the bottle has been opened.

Part II Concentration: A measure of the amount of solute dissolved in

the solution. Molarity (M) Molality (m) Mole Fraction (x) Percent by Mass (%)

Part II

Concentration

Molarity• A unit of concentration of a solution

expressed in moles solute per liter of solution. (Note: 1L = 1 dm3)

Molarity (M) =Moles of Solute

Liters of Solution

Why Molarity ?

http://www.chem.ucla.edu/~gchemlab/volumetric_soln_conc.jpg

Steps involved in the preparation of a standard aqueous solution

Process of making 500 mL of a1.00 M acetic acid solution

Ex. 1 Molarity

What is the molarity of a solution of 8g NaOH in 100mL of solution?

Answer: 2M NaOH

Ex. 2 Molarity

How many grams of NaOH are contained in 2L of a 3M NaOH solution?

Answer: 240g NaOH

Making Dilutions

Moles Solute=M1V1=M2V2

M molarity V volume

Ex. 3 Dilutions

How many milliliters of 2.00M MgSO4 solution must be diluted with water to prepare 100.00 mL of 0.400M MgSO4?

Answer: 20.0mL

Percent by Mass

% by mass= Mass of solute x 100 % Mass of solution

Percent by Volume

% by volume= Volume of solute x 100 % Volume of solution

Part III Molality and Mole Fraction

Molality

Moles of Solute per kilogram of Solvent

Molality (m) = moles solute kg solvent

Mole Fraction

xsolute = Moles of solute Moles of solution

xsolvent = Moles of solvent Moles of solution

xsolute + xsolvent= 1