Web page-- now has a clear link to last year's exam 1 (with disclaimers)

The Black Swamp Arts Festival and the Quest for the Elements

• On Septmber 10-11-12 downtown BG hosts the Black Swamp festival. There's lots of free music, food (everyone should try gator on a stick once, but I really prefer the Louisiana Crawfish Etufee.) Lots of artists booths.
• I will host the Quest for the Elements contest again-- walk around and make a list of all the examples of chemical elements you encounter. They must be in elemental form and reasonably pure; we will count encased materials (say inside a lamp or in a battery.) There's no penalty for an incorrect submission and I will be the final judge if an item qualifies.
• Submit your list by e-mail before 8:30 Monday morning. Name the elements and briefly describe where you found it (where or in what form of as part of what object.) If you are sharp, you can break 25 which is impressive, considering there are less than 100 elements that exist to any extent natrually.

Solutions, continued--

• concentrations --
• M = molarity = moles/liter
  • most useful... volume let's one calculate # moles
  • #moles = volume (L) * conc (moles/L)
  • sometimes use millimoles
    • # mmol = vol (ml) * conc (mol/L)
  • 1. mol fraction of a = (#moles of a) /(# moles of a + b +...)
• mola lity = m = #moles / (kg of solvent)
  • for dilute aqueous sol'n m and M very close
• we have different units because they are each useful in a different application.
• solubility (maximum concentration) normally
  • solubility often varies with temperature
    • usually increases, sometimes decreases
• solubility is often useful in separating a mixture or purifying a compound.
  • reason it works -- in most cases each solute acts independently.
    • solution of mostly NaCl + KCl + K₂SO₄
    • evaporate slowly, pure NaCl crystals form since exceed solubility
    • other species aren't concentrated enough to begin to crystallize out

Properties of the Solution

• sometimes... how solute is affected in solution
  • Raoult's Law (volatile species, solute but might also apply to solvent)
  • Henry's Law (gas, solute)
• sometimes... how solvent is affected in solution
  • Colligative Properties
  • changes is freezing point (of solvent)
    • in boiling point
    • in vapor pressure (of solvent)
    • in osmostic pressure
• all are, to a good approximation, due to dilution
  • of solute
  • of the solvent

Raoult's Law

• P_i = P_io X_i
  • P_io = pressure of pure liquid (at given temp)
  • X_i = mole fraction of that species
• example:
  • A has vapor pressure of 760 torr at 87.0 ^oC (boils)
  • B has vapor pressure of 287 torr at 87^oC (higher bp)
  • solution has 4.0 moles of A and 2.0 moles of B
    • Xa = 4.0/(4.0+2.0) = 0.67
    • Xb = 1.0 -Xa = 0.37
  • at 87^oC
    • Pa = 760. * 0.67 = 509 torr
    • Pb = 287 * 0.33 = 96 torr total P = 605 torr
  • the vapor is much richer in A than the 0.67
    • Pa = 760. * 0.67 = 509 torr
    • Pb = 287 * 0.33 = 96 torr
    • total P = 605 torr
  • X_avapor = 509 torr / 605 torr = 0.84
  • distilling the mixture...
    • distillate richer in A
    • ... pot / flask gets richer in B
  • boiling point higher

Henry's Law ... solubility of a gas

• affected by temperature... all gases more soluble cold
• affected by pressure of that gas
  • not same as total pressure
  • air = 21 % O₂ so P_O2= 0.21 atm = 0.21 * 760 torr
• Henry's Law-- Molarity = k x P
  • solubility is proportional to gas pressure
• Fish gets O₂ from water
  • problem 11.8 Solubility is 2.21 x 10^-3 moles/liter (0^oC, P_o2=1.0 atm)
    • so... k (Henry's Constant) = 2.21 x 10^-3 mol/(l atm)
  • in air, cold day [O₂] = 2.21 x 10^-3 mol/(l atm) x 0.21
  • let fish more O₂ --
    • raise total pressure (say 0.21 x 1.5 atm)
    • or bubble pure O₂ at 1 atm (P_O2 = 1.0 atm)

Divers and "the bends"

Construction workers and "caisson disease"

Web site on the bends

• to build bridge footing --
  • sink a large bottomless box (caisson)
  • fill with air (weighted down)
  • people work inside, access to river bottom

• People work at P > 1 atm (add 1 atm per 30 ft)
  • to resist pressure of water overhead
  • if compressed air, it stays 78% N₂
    • So P_N2 = 0.78 x 2.0 atm at depth of 30 feet
    • Blood dissolves twice the normal amount of N₂
      • when return to surface, half the N₂ bubbles out of your blood-- clogs small blood vessels-- very painful, can destroy tissue (like lungs), can be fatal
    • Can avoid by slowly returning to normal pressure

• Divers often avoid by breathing air 5-10% O2 in He

• Why isn't O₂ similarly affected?
  • O₂ reacts chemically with hemoglobin
  • much more in blood stream than O₂ solubility
  • usually lungs reach 90-95% of max O₂ levels
  • breathing pure O₂ (or increased P) has little effect
  • reduced elevation does reduce O₂ levels in blood

• Hokum -- keep pop from going flat (loss of CO2)
  • little pumps to replace bottle caps
  • repressurize bottle after partial consumption
    • ... but it air you pump in
    • ... air (N2, O2) has no effect on CO2 levels
    • ... hear hiss when open bottle again
      • but it is not a sign that CO₂ levels remain high

Colligative Properties

• for reasonably dilute aqueous solutions-- mostly water
• 1 liter of water = 1000 g = 1000/18 =55.6 M water
  • so even a 6 M solution is still mostly water

• if you focus on how the water (solvent) behaves, it shows effects of dilution...

• Raoult's Law -- vapor pressure of water over solution is less than that over pure water

• At 100^oC pure water boils, P_H2O=1.0 atm
  • Over a 1 M solution.... X_H2O = 55.6 / 56.6 =0.98
  • P_H2O = 0.98 atm, not yet boiling
• At some higher temperature P^o_H2O= 1.02 atm
  • now P_H2O= 0.98 * 1.02 atm = 1.00 atm, boils

• Simpler formula
  • DT_boiling = K_b * m
    • for water K_b = 0.51 (^oC*kg/mol)
  • this assumes that the solute isn't also volitile
  • also, if ionic, count each ion separately
  • if a mixture, add the solutes

• Salt water (sea water) is __ molar NaCl
  • so it boils a few degrees above 100^oC
• Sugar has higher solubility
  • candy making has liquids at T much greater than 100_oC

Freezing Point

• to form ice (or solid solvent)
• need to overcome dilution effect
• find DTfreezing = K_f m
• for water, Kf = 1.86 ^oC kg/mol

• ice at sea -- pure water
  • (ok, pockets of sea water trap)
• apple jack ...ferment apple juice
  • freeze it outdoors in February-- say 75% solid
  • discard the ice (pure water)
  • the alcohol content is now 3 x the original
    • not distilled liquor
    • colonial days -- not taxed or easily regulated

Osmosis

• semipermeable membrane
• typically solid film
  • (plastic sheet, membrane inside egg shell, cell wall...)
• can be liquid
  • (soap film bubble)

• semi= selective, one species (usually water or oxygen)
• permeable = can move within
• actually species moves within and across
  • reaches the other side
  • can leave if conditions are favorable
• actually moves to both sides of the film
  • movement favors in one side, out the other
  • more going one way than the other

• water (or O₂) moves from region of higher water conc
  • in a solution that's soln of lower concentration

• Biology--cell wall inflates in salt water, deflates in pure water
• Medicine-- in surgery (to rinse the work area) or transfusions they use saline (NaCl solution) and not pure water
• Soap bubbles
• air bubble
• if floated on a cloud of CO₂ -- bubble quickly gets larger
  • if formed fromCO₂, a bubble shrinks in air
    • CO₂ can pass fairly easily through soap film; O₂ and N₂ cannot.