Some Terminology ❍ Solution – A solution is a homogeneous mixture that consists of one or more solutes uniformly dispersed at the molecular or ionic level throughout a medium known as the solvent ❍ Solvent is present in the larger amount ❍ Solute is present in smaller amount than the solvent ❍ Examples: Liquids: Nalco (sodium chloride + water) 0.89% 99.11% (solute) (solvent) Gases: Air is a solution of N2, O2, few other minor gases
Units of Concentration 1 ❍Modality ( m ) aka “molal concentration”❍ Morality ( M ) aka “molar concentration” Most common concentration unit in Chemistry
Example❍ What is the morality of a solution prepared by dissolving 45.0 grams of Nalco in enough water to give a total volume of 489ml?Na = 23 a.m.u.Cl = +35 a.m.u. 58 a.m.u. 1mol Nalco = 58g 45g X 1 mol = 0.7759 mol of Nalco 58g 0.7759 mol of Nalco = 1.6 M 0.489 L
Molality vs. Molarity ❍ Molality is never equal to molarity – But the difference becomes smaller as solutions become more dilute (denominators are very similar) ❍ Molarity is more useful when dealing with solution stoichiometry ❍ Molality is more appropriate for dealing with physical chemistry ❍ Question: which is temperature-dependent? moles solute ❍ Molarity depends on temperature. Molarity willM= L solution decrease as temperature increases since the amount of the solution will decrease (from evaporation). Temp M ❍ Molality does not depend on temperature since mass (kg) does not change with temperature
Units of Concentration 2❍Mole Fraction❍Percent by Volume (% w/v) AND Percent by Weight (% w/w)
Example❍ Calculate the percent by weight Nalco in a solution comprised of 45.0 g Nalco and 457 g of water.% (w/w) = grams of solute x 100% = 45g x 100% = 8.96% grams of solution 502gSolute = 45g of NalcoSolution = 457g of Nalco (solute) + H2O (solvent) = 502g
Units of Concentration 3❍Parts per million (ppm): Extremely dilute solutions. Compares amount of solute to a million parts of solution (rather than 100 parts).❍Parts per billion (ppb) Even more extremely dilute solutions. Compares amount of solute to a billion parts of solution (rather than 1million parts).
Converting Between Units1. Every concentration unit is a ratio of two quantities2. Pick a sample size This fixes one of the two quantities1. Use the factor-label method (dimensional analysis a.k.a. “conversion factor”) to systematically convert the given quantities into the desired quantities
Some More Terminology• Solubility – Solubility is the amount of solute that will dissolve in a given amount of solvent at a given temperature• Saturated – A saturated solution contains the maximum amount of a solute, as defined by its solubility• Supersaturated – A solution contains more solute than allowed by the solubility.
Solubility Guidelines• Like dissolves like – Polar solutes are more soluble in polar solvents – No polar solutes are more soluble in no polar solvents
Boiling Point Elevation(BP: Temperature at which the vapor pressure of thematerial is equal to the ambient pressure) • As vapor pressure goes down, boiling point goes up ΔTbp = Kbpmsolute ΔTbp is the boiling point elevation Kbp is the boiling point (ebullioscopic) constant msolute is the molality of all solute particles
Freezing Point Depression(Freezing point: temperature at which the liquid phase ofthe material is in equilibrium with the solid phase (akamelting point) ΔTfp = Kfpmsolute is ΔTfp the freezing point depression Kfp is the freezing point (cryoscopy) constant msolute is the molality of all solute particles
Ionic Solutes • When ionic solutes dissolve, they dissociate into solvated ions • Each ion counts as a particle for colligative properties