Chapter 13 Solutions

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Chapter 13Solutions
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Liquids

• Miscible means that two liquids can dissolve in
  each other
• water and antifreeze, water and ethanol
• Partially miscible- slightly
• water and ether
• Immiscible means they cant
• oil and vinegar

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• In general the principle concerning whether
  substances will mix is "Like dissolves Like"
• that is, polar (and ionic) substances will
  dissolve in polar solvents and non-polar
  substances will dissolve in non-polar solvents,
• while non-polar and polar substances will not
  mix.

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Solubility in Water The simple rule to remember
is like dissolves like
2?-
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Water is a polar solvent and will dissolve polar
molecules and substances that contain charged
particles.
Substances that dissolve in H2O are said to be
soluble E.g. Sugar, ethanol which are
polar most ionic compounds
Site of polarity can form H-bonding
sucrose
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Water is polar, the different ends are attracted
to the charged ions.
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2?-
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Cl-
Na
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Substances that dont dissolve are called
insoluble E.g. Petroleum (crude oil), which are
non-polar So if you want to dissolve grease
which is non-polar, you need to use a non-polar
solvent.
Petroleum in a non-polar organic molecule
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Solution formation

• Nature of the solute and the solvent
• Whether a substance will dissolve
• How much will dissolve
• Factors determining rate of solution...
• stirred or shaken (agitation)
• particles are made smaller
• temperature is increased

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Temperature and Solutions

• Higher temperature makes the molecules of the
  solvent move around faster and contact the solute
  harder and more often.
• Speeds up dissolving.
• Usually increases the amount that will dissolve
  (exception is gases)

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How Much?

• Solubility- The maximum amount of substance that
  will dissolve at a specific temperature (g
  solute/100 g solvent)
• Saturated solution- Contains the maximum amount
  of solute dissolved
• Unsaturated solution- Can still dissolve more
  solute
• Supersaturated- solution that is holding more
  than it theoretically can seed crystal will make
  it come out

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Concentration is...

• a measure of the amount of solute dissolved in a
  given quantity of solvent
• A concentrated solution has a large amount of
  solute
• A dilute solution has a small amount of solute
• thus, only qualitative descriptions
• But, there are ways to express solution
  concentration quantitatively

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• Concentration
• amount of solute
• amount of solvent (or solution)

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Molarity

• (M) amount of solute in moles
• amount of solution in liters

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What is the molarity of a solution with 2.0 moles
of NaCl in 250 mL of solution?

• Molarity moles/liter
• 2.0 mol/ .250 L
• 8.0 M
• ( 8.0 mol/l 8.0 molar are other units that
  can be used.)

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Calculating molarity

• Calculate the molarity of Na2SO4 (s) if you have
  4.2 moles in 500. L of solution.
• Molarity moles/liter
• 4.2 mol/ 500. L
• 8.4 x 10-3 M
• Could also have mol/l or molar as units

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• How many moles of NaCl are needed to make 6.0 L
  of a 0.75 M NaCl solution?
• Moles (0.75 mol/l)(6.0 l)
• 4.5 mol

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Calculate the mass of Na2 SO4 (s) in 1.6 L of a
2.5 M solution.
moles
Then Find mass
Mole x molar mass
M x V

• Molarity moles/volume
• Moles (molarity)(volume)
• ( 2.5 mol/l) ( 1.6 l)
• 4.00 mol

mass
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• Need molar mass 142.02 g/mol

Mass 568.08g 570 g
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Making solutions

• 10.3 g of NaCl are dissolved in a small amount of
  water, then diluted to 250 mL. What is the
  concentration?
• How many grams of sugar are needed to make 125 mL
  of a 0.50 M C6H12O6 solution?

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Alternate Measures of Concentration

• Molar concentration is the accepted method of
  determining the amount of substance dissolved in
  a solvent.
• parts per million (ppm),
• parts per billion (ppb)
• parts per trillion (ppt).

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• Part per million
• ppm mass of solute x 106
• mass of solution
• Part per billion
• ppb mass of solute x 10 9
• mass of solution
• Part per trillion
• ppt mass of solute x 10 12
• mass of solution

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Molality
Molality moles of solute kg of solvent

• (independent of temperature)
• 1 kg of water 1 liter of water

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Molality

• - What is the molality when 0.750 mol is
  dissolved in 2.50 L of solvent?
• Molality 0.750 mol
• 2.50 kg
• 0.300m

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Molality

• Ex- Suppose you had 58.44 grams of NaCl and you
  dissolved it in exactly 2.00 kg of pure water
  (the solvent). What would be the molality of the
  solution?

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• Step One convert grams to moles.
• Step Two divide moles by kg of solvent to get
  molality.
• 58.44 grams/mol is the molecular weight of NaCl
• 58.44g x 1 mol 1.00 mol.
• 58.44 g
• Molality 1.00 mol
• 2.0 kg
• 0.500 mol/kg (or 0.500 m). or 0.500-molal.

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• 1) Calculate the molality of 25.0 grams of KBr
  dissolved in 750.0 mL pure water. (0.280m)
• 2) 80.0 grams of glucose (C6 H12 O6, mol. wt
  180. g/mol) is dissolved in1.00 kg of water.
  Calculate the molality. (0.444m)
• 3) Calculate the molality when 75.0 grams of
  MgCl2 is dissolved in 500.0 g of solvent.(1.58m)

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Mass Percent
1. We can describe the concentration of a
solution by using the mass percent. Mass percent
is the mass of solute present in a given mass of
solution.
Mass Percent mass of solute
? 100 Mass of solution grams of solute
? 100 (grams of solute) (grams of
solvent)
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Example 1 We make a solution by dissolving 1.0g
of sodium chloride in 48g of water. What is the
mass percent of the solute?
Mass Percent 1 .0 g solute x100 49 g
solution
2.0 NaCl
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Example 2 A solution is prepared by dissolving
1.00g of ethanol, C2H5OH, with 100.0g of water.
What is the mass percent of ethanol in this
solution?
Mass Percent 1 .00 g ethanol ?100
101 g solution
0.990 C2H5OH
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Example 3 Cows milk typically contains 4.5 by
mass of the sugar lactose, C12H22O11. Calculate
the mass of lactose present in 175 g of milk.
Mass of solution (milk) 175g Mass percent of
solute (lactose) 4.5
Mass Percent grams of solute
? 100 grams of solution
4.5 grams of solute ? 100 175 g
Grams of solute (4.5)?(175 g) 0.045
? 175g 100
Grams of solute 7.9g lactose