AQUEOUS SOLUTIONS

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					AQUEOUS SOLUTIONS

Solute Concentration

Molecular Weight = Sum of weight of all atoms in a
molecule (expressed in Daltons).
For example: Determine a mole of CH3COOH
  CH3COOH                 2C        2 x 12 Da = 24
                          4H        4 x 1 Da =        4
                          2O        2 x 16 Da = 32
                                 ---------------------
                                 M.W.      Da = 60 g/mol
AQUEOUS SOLUTIONS

Mole = Amount of a substance that has a mass in grams
numerically equivalent to its molecular weight in Daltons.
For example: To determine a mole of sucrose
  (C12H22O11).
Calculate molecular weight:
C = 12 Da         12 Da x 12 = 144 Da
H = 1 Da           1 Da x 22 =       22 Da
O = 16 Da         16 Da x 11 = 176 Da
                                    342 Da
Express it in grams (342 g).
AQUEOUS SOLUTIONS

Molarity = Number of moles of solute per liter of solution.
For example: To obtain 1 M sucrose solution, weigh out
              342 g and add water up to 1L.
Advantage of measuring in moles:
1. Rescales weighing single molecules in Daltons to grams,
   which is more practical for laboratory use.
2. A mole of one substance has the same number of
   molecules as a mole of any other substance
   (6.02 x 1023; Avogadro’s number).
3. Allows one to combine substances in fixed ratios of
   molecules
AQUEOUS SOLUTIONS

For example:
     How much CH3COOH do you need to make 1 liter of a
     2 M solution of CH3COOH?

Ask yourself these questions:
1. How many grams of this substance are in
   1 mole of CH3COOH?
2. How many liters are you trying to prepare?
3. What is the ultimate concentration you are
   trying to prepare (M)?
AQUEOUS SOLUTIONS


Answer:

60 grams/mole x 1 liter x 2 moles/liter =
120 g of CH3COOH is needed to prepare a
2 M solution.
AQUEOUS SOLUTIONS

For Example:

How much CaCl2 do you need to make 750 ml of
a 1.5 M CaCl2 solution?
      How many grams/mole? 110 g/mol
      How many liters? .750 L
      What is M? 1.5 M
AQUEOUS SOLUTIONS

Answer:

110 g/mole x .750 L x 1.5 moles/liter =
123 g

You will need 123g of CaCl2
AQUEOUS SOLUTIONS

For Example:
  You want to make a 10 M C6H12O6 solution. If
  you have 900 grams of C6H12O6, how much can
  you make?
       How many grams/mole? 180 g/mol
       What is the M desired? 10 M
       How many grams total do you have to
       work with? 900 grams
AQUEOUS SOLUTIONS

Answer:

     You can make 0.5 L of a
     10 M C6H12O6 solution
AQUEOUS SOLUTIONS


What if the question would have asked for
a 2.5 M C6H12O6 solution? What about
a 4 M C6H12O6 solution?
AQUEOUS SOLUTIONS

Answer:

5 moles / 2.5 moles/liter = 2 L
5 moles / 4 moles/liter = 1.25 L
AQUEOUS SOLUTIONS
For Example:
  What is the molar concentration of a NaOH
  solution where you have been given 60 grams of
  NaOH and ask to prepare a final volume of
   3 liters?
         What is the amount of grams/mole? =
       40 g/mole
       How many grams do you have? 60 grams
       What is your desired volume? 3 L
AQUEOUS SOLUTIONS

Answer:

60 grams / 40 grams/mole = 1.5 mole
1.5 moles / 3 liters = 0.5 moles/liter
The molar concentration is 0.5 M.
Acids, Bases and pH
Dissociation of Water Molecules (pH)

Occasionally, the hydrogen atom that is
shared in a hydrogen bond between two
water molecules, shifts from the oxygen
atom to which it is covalently bonded to the
unshared orbitals of the oxygen atom to which it is
hydrogen bonded.
Figure 3.9 The pH of some aqueous solutions
Acids, Bases and pH
1. Only a hydrogen ion (proton with +1
    charge) is actually transferred.
2. Transferred proton binds to an unshared orbital
   of the second water molecule creating a
   hydronium ion (H3O+).
3. Water molecule that lost a proton has a net
   negative charge and is called a hydroxide ion
   (OH-).
    H2O + H2O ↔ H3O+ + OH-
Unnumbered Figure (page 47) Chemical reaction: hydrogen bond shift
Acids, Bases and pH

4. By convention, ionization of H2O is
   expressed as the dissociation
    into H+ and OH-
            H2O ↔ H+ + OH-
5. Reaction is reversible.
6. At equilibrium, most of the H2O is
    not ionized.
Acids, Bases and pH

1. At equilibrium in pure water at 25oC:
2. Number of H+ ions + number of OH- ions.
[H+] = [OH-] =          1____ M = 10 –7 M
                    10,000,000

*Note that brackets indicate molar concentration*
Acids, Bases and pH



A solution in which:
[H+] = [OH-] is a neutral solution.
[H+] > [OH-] is an acidic solution.
[H+] < [OH-] is a basic solution.
Acids, Bases and pH
The pH Scale
In any aqueous solution: [H+] [OH-] = 10-14 M2
In a neutral solution,
      [H+] =10-7 M and [OH-] =10-7 M.
In an acidic solution if the
      [H+] = 10-5 M, then [OH-] =10-9 M.
In a basic solution if the
      [H+] =10-9 M, then [OH-] = 10-5 M.
Acids, Bases and pH
pH = Negative log10 of the [H+] expressed
in moles per liter.
pH of 7 is a neutral solution.
pH < 7 is an acidic solution.
PH > 7 is a basic solution.
Most biological fluids are within the
pH range of 6 to 8. There are some exceptions
such as stomach acid with pH of 1.
Acids, Bases and pH

Each pH unit represents a tenfold difference
(scale is logarithmic ), so a slight change in
pH represents a large change in
actual [H+].
pH = -log [H+] or [H+] = 10 – pH M
pOH = -log [OH-] or [OH-] = 10 – pOH M
               **pH + pOH = 14**
Acids, Bases and pH

For Example :
  If the concentration of OH- in an aqueous
  solution is 10-3, what is the pH?
Acids, Bases and pH

pOH = -log [OH-]      pH + pOH = 14
pOH = -log [10-3]     pH + 3 = 14
pOH = 3               pH = 14 – 3

      Final Answer   pH = 11
Acids, Bases and pH

For Example:
  What is the H+ concentration in a solution
  that has a pH of 7?
Acids, Bases and pH

Answer:

1 X 10   –7   M
Acids, Bases and pH

For example:

How much greater is the [H+] in a solution
with pH 2 than in a solution with pH 6?
Acids, Bases and pH
Answer:

pH 2 = [H+] of 10-2 =   1 M
                       100
pH 6 = [H+] of 10-6 =      1    M
                      1,000,000

       10,000 times greater.
Acids, Bases and pH

Buffers

By minimizing wide fluctuations in pH,
buffers help organisms maintain the pH of
body fluids within the narrow range
necessary for life (usually pH 6-8).
Acids, Bases and pH
Buffer
  1. Substances that prevent large
      sudden changes in pH.
  2. Are combinations of H+ -donor and
     H+ -acceptor forms of weak acids or
     bases.
  3. Work by accepting H+ ions from solution
      when they are in excess, and by donating
      H+ ions to the solution when they have been
      depleted.
Acids, Bases and pH

For example:
         Bicarbonate buffer

 H2CO3              HCO3- + H+

 H+ donor              H+ acceptor
 Weak acid              Weak base
Acids, Bases and pH

HCl + NaHCO3          H2CO3 + NaCl
Strong acid           Weak acid



NaOH + H2CO3      NaHCO3 + H2O
Strong base           Weak base

				
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posted:11/23/2011
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