Properties of Solutions. Overview of factors affecting solubility Ways of expressing concentration Physical properties of solutions

Transcription

1 Properties of Solutions Overview of factors affecting solubility Ways of expressing concentration Physical properties of solutions

2 Learning objectives Define terms solute, solvent and solution Distinguish between solutions and heterogeneous mixtures Distinguish among non-, weak and strong electrolytes (Review) Describe factors that affect solubility Describe Henry s law and its application to explain common phenomena involving gases Perform calculations of solution concentration using various definitions Use molarity in stoichiometry calculations Describe basis of Raoult s law and colligative properties Calculate solute concentrations in colligative property context Explain basis of osmotic pressure

3 Definitions of a solution A homogeneous mixture of two or more substances Solute is the component that is dispersed in the solvent usually the minority component Solvent is the dispersing component usually the majority component Sometimes definitions can become blurred: water (solvent) dissolves much greater than its own mass of sugar (solute)

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5 Formation of solution of ionic compound Crystals are held together by strong ionic bonds Polar water molecules exert attractive forces on ions Hydration of the ions by water molecules overcomes lattice energy Crystal lattice disperses

6 Like dissolves like All gases mix with each because there are no intermolecular forces Solids and liquids mix if intermolecular forces between unlike substances (adhesive forces) are similar to forces between like substances (cohesive forces)

7 Electrolytes redux Electrolytes are substances that dissociate into ions in solution ionic compounds (sodium chloride) Strong electrolytes are completely ionized NaCl s Na aq Cl aq HO 2 ( ) ( ) ( ) Weak electrolytes are partly ionized HO 2 HF( aq) HF( aq) H ( aq) F ( aq) Non-electrolytes are those substances that produce no ions (sugar)

8 Factors affecting solubility Difference in polarity between solute and solvent like dissolves like Temperature Solid solutes: depends on balance of several factors can increase, decrease or stay the same Gases: solubility always decreases with temperature Pressure Solids: little influence Gases: solubility always increases with pressure (Henry s law)

9 Saturation A saturated solution is one which is in equilibrium with undissolved solute it has reached limit of solubility Supersaturation arises when amount of substance in solution is greater than that predicted on basis of saturation. An essential condition for the growth of crystals

10 Henry s Law The number of moles of gas dissolved in a liquid is proportional to the partial pressure of the gas Exchange of CO 2 and O 2 in respiration depends on Henry s Law. In the lungs, the O 2 partial pressure is higher than that of CO 2 In the blood, the CO 2 pressure is higher after respiration

11 Measuring concentration Concentration = amount of solute/amount of solution Weight/weight percent Mass solute in g/mass solution in g x 100% Weight/volume percent Mass solute in g/volume of soln in ml x 100% Parts per million (ppm)/billion (ppb) Mass solute in g/mass solution in g x 10 6 or 10 9 Molarity Moles of solute per liter of solution Molality Moles of solute per kilogram of solvent

12 What is w/w% of a solution containing 40 g ethanol and 60 g water?

13 What is w/v % of 2.5 g NaCl in 250 ml solution?

14 What mass KNO 3 is present in 5 L of a 10 w/v% solution?

15 Molarity Concentration is usually expressed in terms of molarity: Moles of solute/liters of solution (M) Moles of solute = molarity x volume of solution Moles = M x V

16 Molarity and concentration Molarity: Dilution M = moles solute/liter of solution M 1 V 1 = M 2 V 2 Dilution factor = V 2 /V 1 (V 2 >V 1 ) M 2 <M 1

17 Example What is molarity of 50 ml solution containing g H 2 SO 4? Molar mass H 2 SO 4 = 98.1 g/mol Moles H 2 SO 4 = mol Volume of solution = L g 98.1 g/mol 1 L 50 ml x 1000 ml Concentration = moles/volume = M mol L

18 What is concentration of solution containing 60 g NaOH in 1.5 L

19 Dilution More dilute solutions are prepared from concentrated ones by addition of solvent Dilution: V 2 > V 1 Molarity of new solution M1V1 M 2V2 M V V M V V 2 2 M V M V To dilute by factor of ten, increase volume by factor of ten

20 What is concentration if 2 L of 6 M HCl is diluted to 12 L?

21 How much water must be added to make a 2 M solution from 100 ml of 6M solution?

22 Summary of stoichiometry

23 Solution stoichiometry How much volume of one solution to react with another solution Given volume of A with molarity M A Determine moles A Determine moles B Find target volume of B with molarity M B Volume A mol = MV Moles A Mole:mole ratio Moles B V = mol/m Volume B

24 Titration Use a solution of known concentration to determine concentration of an unknown Must be able to identify endpoint of titration to know stoichiometry Most common applications with acids and bases

25 Example How much M NaHCO 3 is required to neutralize 18.0 ml of M HCl?

26 Colligative properties Properties that depend upon the concentration of solute particles but not their identity Vapor pressure lowering Freezing point depression Boiling point elevation Osmotic pressure

27 Raoult s law When nonvolatile solute is added to solvent, vapor pressure of solvent decreases in proportion to concentration of solute Freezing point goes down Boiling point goes up

28 Adding salts upsets the equilibrium Fewer water molecules at surface: rate of freezing drops Ice turns into liquid Lower temperature to regain balance Depression of freezing point

29 The same model explains elevated Condensation and evaporation are dynamic processes Replacing some of the liquid water with salt reduces rate of evaporation leads to condensation Raise temperature to recover balance boiling point

30 Mathematical base Freezing point depression ΔT f = k f x solute concentration Boiling point elevation ΔT b = k b x solute concentration

31 Units of concentration Effect depends upon number of particles not mass of particles, so concentration must be in moles. Molality (m) is used in these situations Moles solute/kg solvent Temperature independent measure of concentration

32 Calculate molality of solution made from 60 g NaOH and 500 ml water

33 Type of solute important Covalent solute produces one particle per molecule: C 6 H 12 O 6 (s) C 6 H 12 O 6 (aq) Ionic solutes produce >1 particle per formula unit: NaCl (s) Na + (aq) + Cl - (aq) (2 particles) CaCl 2 (s) Ca 2+ (aq) + 2Cl - (aq) (3 particles)

34 Osmotic pressure Transport across semipermeable membranes: Solvent particles admitted but solute particles rejected Osmosis involves passage of water molecules across a membrane

35 Osmosis Transport of water molecules from dilute solution to more concentrated one Imbalance of concentration provides driving force Osmotic pressure is the pressure required to oppose this flow

36 Osmotic pressure Osmotic pressure is written as: πv = nrt Osmotic pressure Volume Tempera ture No moles Gas constant

37 Calculating osmotic pressure But n/v = concentration in moles per liter =M π = MRT (T in Kelvin) But what molarity? Need to know moles of particles C 6 H 12 O 6 = 1 mole particles NaCl = 2 moles of particles CaCl 2 = 3 moles of particles Osmolarity refers to concentration of particles for osmotic pressure determination

38 Osmotic pressure and cells Concentration in cells depends on osmosis Concentration outside cell > inside (hypertonic) crenation Concentration outside cell < inside (hypotonic) - hemolysis Crenation Hemolysis