CH 104: SOLUBILITY AND SOLUTIONS

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CH 104 SOLUBILITY AND SOLUTIONS

• In todays experiment you will measure the
  molarity (M), molality (m), mole fraction (?),
  and mass percent of a potassium chloride (KCl)
  solution.
• What is molarity (M)?
• What is molality (m)?
• What is mole fraction (?)?
• What is mass percent?
• Which, if any, of these concentrations will
  change with temperature?
• Only molarity (M) will change with temperature.
  The volume of solution changes with temperature.
  In contrast, the moles of solute, mass of solute,
  moles of solvent, and mass of solvent do not
  change with temperature.

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SOLUBILITY AND SOLUTIONS

• A solution is made by dissolving 1.370 grams (g)
  of KCl in 100.0 g of water. Its final volume is
  100.9 milliliters (mL). What are the molarity
  (M), molality (m), mole fraction of KCl (?KCl),
  and mass percent of KCl for this solution?

You just calculated 4 different concentrations of
KCl for the same solution. Each has its
advantages and disadvantages.
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SOLUBILITY AND SOLUTIONS

• The terms insoluble, slightly soluble, soluble,
  and very soluble describe the relative solubility
  of a substance in a solvent.
• In todays experiment you will use these terms to
  compare the solubilities of sodium carbonate
  (Na2CO3), naphthalene (C10H8), and calcium
  carbonate (CaCO3) in 2 different solvents.
• The first solvent is water (H2O) and the second
  solvent is cyclohexane (C6H12). Water is
  relatively polar and C6H12 is relatively
  nonpolar.
• Like dissolves like. That is, polar solvents
  tend to dissolve polar solutes. And nonpolar
  solvents tend to dissolve nonpolar solutes.

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SOLUBILITY AND SOLUTIONS

• Describe the electrostatics for the dissolution
  of an ionic crystal in water.
• Water dipoles cluster around ions at the surface
  of the crystal. The negative ends of these
  dipoles are oriented toward the positive ions
  (cations). In contrast, the positive ends of
  these dipoles are oriented toward the negative
  ions (anions). The solid will dissolve if these
  ion-dipole forces are greater than the interionic
  attractions within the crystal. Moreover, these
  ion-dipole forces persist in the solution. An
  ion is hydrated when it is surrounded by water
  molecules.

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SOLUBILITY AND SOLUTIONS

• The terms immiscible, partially miscible, and
  infinitely miscible describe the relative
  solubility of a liquid solute in a liquid
  solvent.
• In todays experiment you will use these terms to
  compare the miscibility of water (H2O),
  2-propanol (isopropyl alcohol, CH3CHOHCH3), and
  cyclohexane (C6H12). Water is the most polar and
  C6H12 is the least polar. Why?
• Water has 2 hydrogens bonded to oxygen.
  CH3CHOHCH3 has 1 hydrogen bonded to oxygen. And
  C6H12 has no hydrogens bonded to oxygen. These
  OH groups are polar and form hydrogen bonds with
  other molecules.
• The hydrogen bonds of water are shown as dotted
  lines.

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SOLUBILITY AND SOLUTIONS

• A corollary to like dissolves like is oil and
  water dont mix.
• Oil and water in a separatory funnel.

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SOLUBILITY AND SOLUTIONS

• Why doesnt oil and water mix?
• The hydrogen bonds of water are shown as dotted
  lines.
• Water is an associated liquid that is, adjacent
  water molecules are held together by relatively
  strong hydrogen bonds. In contrast, oils and
  other nonpolar liquid are held together by
  relatively weak intermolecular forces.
  Therefore, the attraction between oil and water
  molecules is not strong enough for the oil to
  break the hydrogen bonds of water and dissolve
  into the water.

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SAFETY

• Do NOT use cyclohexane (C6H12) or 2-propanol
  (CH3CHOHCH3) until all the Bunsen burners in the
  laboratory are extinguished. Treat these
  compounds as if they were gasoline. They are
  extremely flammable.
• Wear your goggles at all times.
• Your laboratory manual has an extensive list of
  safety procedures. Read and understand this
  section.
• Ask your instructor if you ever have any
  questions about safety.

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SOURCES

• McMurry, J., R.C. Fay. 2004. Chemistry, 4th ed.
  Upper Saddle River, NJ Prentice Hall.
• Petrucci, R.H. 1985. General Chemistry Principles
  and Modern Applications, 4th ed. New York, NY
  Macmillan Publishing Company.
• Wright, A.E. 2006. NOAA Ocean Explorer.
  Available http//www.oceanexplorer.noaa.gov/explo
  rations/02sab/background/products/products.html
  accessed 13 December 2006.