Unit 8 Solutions

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Unit 8 Solutions

• Textbook Ch. 18
• RB Topic 7

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I. Solutions

• A solution is a particular type of mixture or
  combination of different substances
• Each substance retains its chemical properties
  (they are combined physically, not chemically)
  and
• The substances can be separated by non-chemical
  means
• filtration, evaporation, separatory funnel,
  distillation, or centrifugation.

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Solutions continued

• A solution has two components the solute and the
  solvent.
• The solvent is the substance in greater amount.
• It is usually a liquid, although it does not have
  to be.
• It is usually water, but it does not have to be.

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Solutions continued

• The solute is the substance in lesser amount.
• It is usually a solid, although it does not have
  to be.
• The occasional liquid solute will be mentioned.
• Most often solutes are solid ? think U are solid

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Solutions continued

• A solution is a homogeneous mixture
• Homogeneous means that the mixture is the
  ________ all the way through.
• You could take two same-sized samples one from
  the bottom and one from the top and they would be
  __________.
• Homogeneous mixtures do not settle out, a
  heterogeneous mixture would.
• Blood is a heterogeneous mixture.

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II. Forming Solutions

• Solutions form at different rates
• Rate depends on 3 factors
• Agitation (stirring)
• Temperature (?KE)
• Surface area (particle size)
• The key is contact between solute and solvent

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Forming Solutions

• The more surface area exposed to solvent, the
  faster the solute will dissolve
• Also, increased movement and temp will increase
  collisions and speed up the rate

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III. Types of Solutions

• Solid dissolved in solid (metal in metal ? alloy)
  Ex. brass
• Gas dissolved in gas (air mixture of 02, N2, CO2,
  etc)
• Liquid/Solid/Gas dissolved in a liquid

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Types of Solutions

• Liquids dissolved in liquid
• If liquids can dissolve in each other ? Miscible
  (like dissolves likes)
• Ex. Ethanol and Water (polar)
• If liquids cannot dissolve in each other?
  Immiscible (unlike)
• Ex Oil (nonpolar) and Water (polar)

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Types of Solutions

• Solids
• If solid can dissolve in liquid ? Soluble
• If solid cannot dissolve in liquid ? Insoluble

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IV. Solubility

• Solubility the amount that dissolves in a given
  quantity of a solvent at a given temperature to
  produce a saturated solution
• In other words the solubility is how much will
  dissolve

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Solubility

• Saturated solution contains all the solute it
  can hold at a given temperature
• Unsaturated contains less solute than it can
  hold under existing conditions

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Solubility

• Supersaturated contains more solute than it
  would normally hold at a given temperature
  usually not stable, may cause solution to
  crystallize and precipitate out of solution
• Dilute or Concentrated does not indicate if a
  solution is saturated or unsaturated!

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Solubility factors

• Temperature affects solubility
• Usually an increase in temp, increases solubility
  ? solids
• There are exceptions
• Yb (ytterbium) and gases
• Also, NaCl only increases slightly

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Solubility factors

• As gas temp increases, solubility decreases ?
  more KE causes dissolved gas particles to escape
  thus decreasing solubility
• Increased partial pressure, however, will
  increase solubility of gases
• What is partial pressure?
• The pressure exerted by a gas

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Solubility factors

• Considering Henrys Law, why will soda become
  flat after opening?
• Partial pressure and Henrys Law will not be on
  the Regents exam
• Stay tuned for molarityit will be!

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V. Molarity

• The amount of solute that will dissolve in a
  solvent depends on the nature of both substances
  and on the temperature.
• A solution with very little dissolved solute is
  said to be dilute
• A solution with a large amount of dissolved
  solute is said to be concentrated
• These are qualitative descriptions only, NOT
  numerical values.

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Molarity

• As is clear from its name, molarity involves
  moles.
• The molarity of a solution is calculated by
  taking the moles of solute and dividing by the
  liters of solution.

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Molarity

• Example 1 - Suppose we had 1.00 mole of sucrose
  (it's about 342.3 grams) and proceeded to mix it
  into some water. It would dissolve and make sugar
  water. We keep adding water, dissolving and
  stirring until all the solid was gone. We then
  made sure that when everything was well-mixed,
  there was exactly 1.00 liter of solution.
• What would be the molarity of this solution?

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Molarity

• The answer is 1.00 mol/L. Notice that both the
  units of mol and L remain. Neither cancels.
• A replacement for mol/L is often used. It is a
  capital M. So if you write 1.00 M for the answer,
  then that is correct.
• And never forget this replace the M with mol/L
  when you do calculations. The M is just shorthand
  for mol/L.

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Molarity

• Example 2 - Suppose you had 2.00 moles of solute
  dissolved into 1.00 L of solution. What's the
  molarity?
• The answer is 2.00 M.
• Notice that no mention of a specific substance is
  mentioned at all. The molarity would be the same.
  It doesn't matter if it is sucrose, sodium
  chloride or any other substance. One mole of
  anything contains 6.022 x 1023 units.

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Molarity

• Example 3 - What is the molarity when 0.75 mol
  is dissolved in 2.50 L of solution?
• The answer is 0.30 M.
• Now, let's change from using moles to grams. This
  is much more common. After all, chemists use
  balances to weigh things and balances give grams,
  NOT moles.

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Molarity

• Example 4 - Suppose you had 58.44 grams of NaCl
  and you dissolved it in exactly 2.00 L of
  solution. What would be the molarity of the
  solution?
• Two steps
• Step One convert grams to moles.
• Step Two divide moles by liters to get molarity.
• In the above problem, 58.44 grams/mol is the
  molecular weight of NaCl. Dividing 58.44 grams by
  58.44 grams/mol gives 1.00 mol.
• Then, dividing 1.00 mol by 2.00 L gives 0.50
  mol/L (or 0.50 M). Sometimes, a book will write
  out the word "molar," as in 0.50-molar.

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Molarity

• Do examples 5 and 6
• 5) Calculate the molarity of 25.0 grams of KBr
  dissolved in 750.0 mL.
• 6) 80.0 grams of glucose (C6H12O6, mol. wt 180.
  g/mol) is dissolved in enough water to make 1.00
  L of solution. What is its molarity?

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Molarity

• Here are the solutions
• 5) Calculate the molarity of 25.0 grams of KBr
  dissolved in 750.0 mL.
• Note the change from mL to L.

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Molarity

• 6) 80.0 grams of glucose (C6H12O6, mol. wt 180.
  g/mol) is dissolved in enough water to make 1.00
  L of solution. What is its molarity?

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Molarity

• Practice Problems
• 1) Calculate the molarity when 75.0 grams of
  MgCl2 is dissolved in 500.0 mL of solution.
• 2) 100.0 grams of sucrose (C12H22O11, mol. wt.
  342.3 g/mol) is dissolved in 1.50 L of solution.
  What is the molarity?
• 3) 49.8 grams of KI is dissolved in enough water
  to make 1.00 L of solution. What is the molarity?

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Molarity

• http//dbhs.wvusd.k12.ca.us/webdocs/Solutions/Solu
  tions.html

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VI. Properties of Solutions

• Clear, do not disperse light
• May have color (but not always)
• Will pass through a filter
• Will not settle out on standing (not like a
  suspension)
• No definite composition (unlike compounds)

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Properties of Solutions

• Most common solvent is water
• Water solutions are called aqueous (aq)? shows
  substance is dissolved in water
• Particle diameter lt or 1 nm (too small to
  settle out)
• No Tyndall Effect say what?
• Most reactions take place in solutions

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VII. Properties of Other Mixtures

• Suspensions heterogeneous mixtures with
  particles that settle out slowly while standing
• Can be collected using filtration (sulfur and
  salt water)
• Particles exhibit the Tyndall Effect, scattering
  of visible light in all directions
• Ex. Clay and water mixture
• Large particle size (gt 100 nm in diameter)
  compared to solutions

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Properties of Other Mixtures

• 2. Colloid heterogeneous mixture, particles do
  not settle out.
• Intermediate particle diameter ?between 1 nm and
  100 nm
• Particles evenly distributed/dispersed
• Particles do not settle while standing
• Cloudy/milky in appearance when concentrated, but
  clear if dilute
• Particles exhibit the Tyndall Effect
• Ex. Glue, fog, paint, milk, jello

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