Chapter 9 Solution Concentrations and Colligative Properties

% Solutions Mass is usually in grams Make sure that you understand that the denominator is the total mass of solution.

Molarity Most widely used concentration unit [HCl] means concentration of HCl in mol/L Notice volume is total volume of solution

Molality Molality (m) expresses concentration in terms of the mass of the solvent.

Molarity vs Molality Molarity and molality differ in two ways: –Molarity tells you about moles of solute per volume of the entire solution (solute & solvent) –Molality tells you about moles of solute per mass of the solvent Keep in mind that one liter of water weighs one kilogram So for a dilute solution, the amount of solution is about the same as the amount of solvent So for a dilute aqueous solution, molarity and molality are basically the same.

Mole Fraction Mole fraction equals the moles of a solute divided by total moles of solution

Remember Density Density is the measure of mass per unit volume. –Since density relates mass and volume, it is useful if you need to convert between molarity, which deals with volume, and molality, which deals with mass. D = m/vunits usually g/mL or g/cm 3

What do you know if given 12.5 % NaCl? 12.5 g NaCl added to 87.5 g H 2 O 100 g solution 87.5 g solvent H 2 O What can you find? (must be given density) Moles solute; liters of solution; kg solvent Molarity Molality Mole fraction

What do you know if given 3.0 M KCl? 3.0 moles KCl in 1 liter solution What can you find? (need density again) find grams KCl find grams solvent Molality Mole fraction % solution

What do you know if given 2.7 m NaOH? 2.7 moles NaOH in 1 kg water What can you find? (need density) grams NaOH mole fraction % solution Molarity

Effect of temperature on solubility Most solids increase solubility when solution is heated…some do not All gases decrease solubility when solution is heated.

Like Dissolves Like Polar or ionic solutes dissolve in polar solvents –When ionic solutes dissolve they break up into ions Nonpolar solutes dissolve in nonpolar solvents. Polar and nonpolar do not mix

Van’t Hoff factor (i) The Van’t Hoff factor tells how many ions one unit of a solute will dissociate into in solution. –C 6 H 12 O 6 does not dissociated, so i = 1 –NaCl dissociates into Na + and Cl -, so i = 2 –CaCl 2 dissociates into Ca 2+, Cl -, and Cl -, so i = 3 –HF partially dissociates, so 1 < i < 2

Boiling Point Elevation When a solute is added to a solvent, the boiling point of the solution increases (that is, bp is higher than that of the solvent) ΔT b = ik b m ΔT b = change in boiling point i = Van’t Hoff factor k b = boiling point elevation constant m = molality of solution

Freezing Point Depression When a solute is added to a solvent, the freezing point of the solution decreases (that is, fp is lower than that of the solvent) ΔT f = ik f m ΔT f = change in freezing point i = Van’t Hoff factor k f = freezing point depression constant m = molality of solution

Vapor Pressure Lowering Called Raoult’s Law – when a solute is added to a solution, the vapor pressure will decrease. P = XP° P = vapor pressure of solution P = vapor pressure of solvent X = mole fraction of solvent

Osmotic Pressure When a pure solvent and a solution are separated by a membrane that only allows solvent to pass through, the solvent will try to pass through the membrane to dilute the solution. The greater the concentration of solute in the solution, the greater the osmotic pressure. Ώ =λ = MRTi Ώ =λ = osmotic pressure (atm) M = molarity R = gas constant =.0821 (L·atm)/(mol·K) T = Kelvin temperature i = Van’t Hoff factor

What do you need to know for test? Like dissolves like Affect of temperature on solubility Molarity Molality Mole fractions % solutions Colligitave properties (math)