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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.
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