DETERMINING THE HYDROGEN ION CONCENTRATION OF STRONG AND WEAK ACIDS

Per-Lab Discussion
When HCl gas dissolves in water, it completely ionizes, as shown by the equation for its ionization:
     HCl  +  H2O  ----->  H3O+  +  Cl-

 The single arrow pointing to the right shows that HCl is a strong acid that completely ionizes.  This means that if 1 mole of HCl dissolves in water to form 1 dm3 of solution, that solution will contain 1 mole of hydronium ions and 1 mole of chloride ions.  That is, the concentration of both the hydronium ions and the chloride ions will be 1 mole per dm3, or 1 molar.  Therefore, a 1 molar hydrochloric acid solution is one in which the hydronium ion concentration is 1 molar.
 The behavior of acetic acid contrasts with that of HCl.  If 1 mole of acetic acid is added to water to form 1 dm3 of solution, less than 1 mole of hydronium ions will be formed because acetic acid, a weak acid, does not ionize completely.  This fact is indicated by the double arrow in the equation for its ionization:
    CH3COOH  +  H2O <------> H3O+  + CH3COO-
A 1 molar acetic acid solution, therefore, has a hydronium ion concentration that is less than 1 molar.  This difference between hydrochloric acid and acetic acid needs to be clearly understood as you do this experiment.
 Weak acids--those that do not ionize completely in water solutions--attain an equilibrium between  the ions formed and the un-ionized molecules.  The equilibrium constant for the system is called the ionization constant, and is represented by the symbol Ka.  Ionization constants for acids are used to determine the relative strengths of acids.  The ionization of a weak acid, HA, can be represented by the equation
    HA +  H2O <----->  H3O+   +  A-
The expression for the ionization constant of this acid is:

    Ka  =  [H3O+] [A-]
                    [HA]

 In this experiment, you will prepare some acid solutions of known hydronium ion concentrations, and you will observe the colors of some acid-base indicators in each of these solutions.  These same indicators then will be added to some solutions with hydronium ion concentrations that are not known.  You will compare the colors of the indicators in the solutions of unknown concentration to their colors in the solutions of known concentration.  This comparison will tell you the hydronium ion concentrations in the unknowns.
 From your determination of the hydronium ion concentration in an acetic acid solution, you will calculate the ionization constant of acetic acid.

PROBLEM:  Using acid-base indicators you will determine the hydronium ion concentration of a strong acid and a weak acid.  Using this experimental data you will calculate the ionization constant of a weak acid.

EQUIPMENT:
12 well plate  0.1 M HCl            0.056 M CH3COOH            0.00056 M CH3COOH
methyl orange                            orange IV                            strong acid solution (conc. unknown)
SAFETY:  Handle acid solutions with extreme care.  Rinse any spills with cold water and report them to your teacher.

                               conc #1                     conc #2                   conc #3                  conc #4

          SET   1

          SET   2

          UNKNOWNS

PROCEDURE:
PART A:  PREPARATION OF SOLUTION OF KNOWN HYDRONIUM ION CONCENTRATION.
In this part of the experiment , you will  prepare two identical sets of HCl solution.  Each set  will consist of 2 ml of solution at  four different  concentrations.

1.  CONCENTRATION 1.  Obtain 5 ml of stock solution.  Using a measuring pipet, put 2 ml of
this solution into a clean, well  for  concentration 1 of Set 1. Put another 2 ml into a second well for
concentration 1 of set 2.  Save the remaining 1 ml of the stock solution for making concentration 2.
Clean your pipet.

2. CONCENTRATION 2.  From the 1 ml of 0.1 M HCl remaining from step 1, carefully pipet 0.5 ml
and place it into a 10-ml graduated cylinder add, distilled water until you have a total of 5.0 ml.
Gently  mix the solution with your pipet.  Put  2 ml of this solution into each of two well for concen-
tration 2 of sets 1 and 2.   The 1 ml of 0.01 M HCl that is left over will be used to make concentration 3.  Clean the pipet.

3.  CONCENTRATION 3. From the 1 ml of 0.01 M HCl remaining from step 2, carefully pipet
0.5 ml and place it into a 10-ml graduated cylinder, add distilled water until you have a total of 5.0 ml. Gently mix the solution with your pipet.  Put 2 ml of this solution into each of two well for
concentration 3 of sets 1 and 2.  The 1 ml of 0.001 M HCl that is left over will be used to make
concentration 4.  Clean the Pipet.

4.  CONCENTRATION 4.  From the 1 ml of 0.001 M HCl remaining form step 3, carefully pipet 0.5 ml and place it into a 10-ml graduated cylinder, add distilled water until you have a total of 5.0 ml.  Gently mix the solution with your pipet.  Put 2 ml of this solution into each of two well for concentration 4 of set 1 and 2.  The 1 m of 0.0001 M HCl that is left over can be discarded.  Clean the pipet..

5.  Add one drop of methyl orange indicator to each well of Set 1 and one drop of orange IV indicator to each well in Set 2.  These two sets are your standard acid solutions.  Note their colors in your data table.

PART B:  HYDRONIUM ION CONCENTRATION OF AN UNKNOWN ACID

6.  Obtain 4 ml  of one of the unknown acid solutions.  Make a note of the number or letter on the bottle from which you obtain the acid.

7.  Place 2 ml of the acid in each of the 2 clean, dry test wells.  Add one drop of methyl orange to one of the wells, and one drop of orange IV to the other.

8.  Compare the colors of these samples with the colors of the standard acid solutions and determine the hydronium ion concentration of the unknown acid.  Record your observations.

PART C:  HYDRONIUM ION CONCENTRATION OF A SOLUTION OF ACETIC ACID

9.  Obtain a 4 ml sample of CH3COOH.  Note and record the molarity of your sample.  (Either 0.056M or 0.00056M)

10. Place 2 ml of the acid into each of two wells.  Add one drop of methyl orange to one well and one drop of orange IV to the other.  Compare the colors of these samples with the colors of the standard acid solutions.  Record your observations.

CALCULATIONS
Calculations 1-3 are from part “B”.

1. Write the equation for the ionization of hydrochloric acid.

2.  Indicate which unknown part “B” acid you had and determine the concentration of hydronium ion in your part “B” acid.

3.  What is the concentration of the original acid in part “B”.  Explain your answer.
Questions 4-7 are from part “C”. 

4. Write the equilibrium equation for the ionization of acetic acid.

5. Write the expression for the ionization constant of acetic acid.

6.  Indicate which unknown part “C” acid you had.  Use the expression for Ka from question 5 and the information you obtained in Part C of the “procedure” to calculate the numerical value of the ionization constant of acetic acid.

7.  The accepted value for acetic acid is 1.8 x 10-5.  Calculate the % error.  In the discussion account for the size of you error.

QUESTIONS

1.  Explain why a 0.1 M HCl solution has a hydronium ion concentration of 0.1 M while a .056 M CH3COOH solution has a hydronium ion concentration of about 0.001 M.

2.  If 1 mole of acetic acid is diluted with water to make 1 dm3 of acid solution, you might think that the concentration of acetic acid molecules in the solution is 1 molar.
 a. Why is this not exactly true ??
 b. Why is “1 molar” a close approximation for the concentration of these molecules??