Colligative properties

Solutions Part 4

Electrolytes: a quick review

• Electrolytes form ions in solution.

• Ions allow water to conduct electric current

• Three types of electrolytes: strong , weak, and non-electrolytes.

Strong Electrolytes• Strong electrolytes ionize

completely in water.

• More ions in the water, more conductivity.

• Strong electrolytes include:– Strong acids HCl H+ + Cl-

– Strong bases NaOH Na+ + OH-

– Soluble salts CaCl2 Ca2+ + 2Cl-

Molarity:review• Moles per liter• 1.0 M = 1 mole solute

per 1 liter solution.

• Question• What is the molarity of

1.56 g of gaseous HCl in a 26.8 mL solution?

Mole Fraction

• mole fraction: the number of moles of a component of a solution divided by the total number of moles of all components.

Example:

• 58.5 grams of NaCl are in a one liter solution.

• What is the Molarity of the NaCl?

• 1 M

• 58.5 grams of NaCl are in 1800 grams of water.

• What is the mole fraction of NaCl?

• 1/101

Molality

• Definition: a unit of concentration, defined to be equal to the number of moles of solute divided by the number of kilograms of solvent .

This is not to be confused with Molarity

Why so you add salt to an icy sidewalk?

• It melts the ice.

• How?

• Colligative properties.

• Why so some types of salt seem to work better than melting the ice than others.

• Why?

• Colligative properties.

Colligative properties

• are properties of solutions that depend on the number of molecules in a given volume of solvent and not on the properties (e.g. size or mass) of the molecules.[1] Colligative properties include: lowering of vapor pressure; elevation of boiling point; depression of freezing point and osmotic pressure.

Colligative properties

• How many ions are formed from the dissociation of 1mole of NaCl?

• From CaCl2?

• From NH4Cl

• From H3PO4?

Question

• What is the concentration of each type of ion in the following solutions?

• 0.50M Co(NO3)2

• 1 M Fe(ClO4)3

Answer• Co 2+ 0.5 M

• NO3 - 1.0 M

• Fe 3+ 1 M

• ClO4 - 3M

• Why isn’t it 3M Cl- and 12 M O2- ??

Vapor Pressure Reduction

• The particle act to attract the water molecules making it more difficult for them to free themselves to become vapor.

Freezing point depression

• The individual particles interfere with the water’s ability to form a crystal depressing the freezing point.