Aim: How does the addition of a solute affect the colligative properties of the solvent? Do Now: What are the units for molarity? How much NaCl in mol does 1 L of a 2M solution contain? What is the volume in liters of a 2 M solution that contains 3 mole of cobalt chloride? What mass of NaF is required to make 0.5 L of a 1 M solution. The molar mass of NaF is 42 g/mol?

Molecules vs. Ionic Compounds in Aqueous solutions Some molecular compounds dissolve but do not dissociate into ions. C6H12O6(s) (glucose)  C6H12O6 (aq) 1 mole of sugar gives 1 mole of sugar when dissolved in water

Molecules vs. Ionic Compounds in Aqueous solutions Many ionic compounds dissociate into independent ions when dissolved in water NaCl (s)  Na+(aq) + Cl-(aq) 1 mole of NaCl solid gives 1 mole of sodium ions and 1 mole of chloride ions when dissolves (total of 2 moles of dissolved particles). MgCl2 (s)  Mg+2(aq) + 2Cl-(aq) Total of 3 moles of dissolved particles

Colligative Properties Colligative properties refers to properties of a solution that depend on the concentration of particles. Vapor pressure Boiling point Freezing point

Vapor Pressure Lowering The presence of any solute (salt or sugar) lowers the vapor pressure of the solvent. The more moles of dissolved particles (greater concentration of solute), the lower the vapor pressure.

Boiling Point Elevation The presence of a nonvolatile solute (salt or sugar) raises the boiling point of the solvent. Nonvolatile: does not easily vaporize The greater the concentration ( more moles of dissolved particles)of the solute, the more it raises the boiling point.

Boiling Point (cont.) The elevation of the boiling point depends on the number of mols of particles present. For each mol of particles in a kg of water the boiling point of the aqueous solution is elevated by 0.52°C. That is called the boiling point elevation constant for water (0.52°C/m). Molecular compounds such as C6H12O6 do not break up into ions, so one mol of C6H12O6 would raise the boiling point 0.52°C. Ionic Compounds such as NaCl and CaCl2 break up into ions in aqueous solution. One mol of NaCl actually yields two mols of ions, one mol of CaCl2 yields three mols of ions. They would elevate the boiling point more than one mol of C6H12O6.

Freezing Point Depression The presence of any solute (salt or sugar) lowers the freezing point of the solvent. The more moles of dissolved particles (greater concentration of solute), the lower the freezing point.

Question How are the boiling and freezing points of a sample of water affected when a salt is dissolved in the water? Boiling point decreases and freezing point depression Boiling point decreases and freezing point increases Boiling point increases and freezing point decreases Boiling point increases and freezing point increases

Question Which solution containing 1 mole of solute dissolved in100 grams of water has the lowest freezing point? KOH (aq) C6H12O6 (aq) C2H5OH (aq) C12H12O11 (aq)

Summary The addition of solutes of water: Lowers vapor pressure Raises boiling point Lowers freezing point The more particles dissolved the greater change it has on the colligative properties.

Other things to remember Low vapor pressure=high boiling point High vapor pressure = low boiling point The stronger the intermolecular forces the lower the vapor pressure/higher the boiling point.