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1 6 D E T E R M I N I N G D I S S O C I A T I O N C O N S T A N T S — L A B A C T I V I T Y 16 Determining Dissociation Constants
Background
Dissociation is the term used to describe the breakdown of a molecule into simpler parts. This often (almost always in basic chemistry) occurs in water. The equilibrium constant for such a reaction is called a dissociation constant. The dissociation constant of a compound is important for a number of reasons: The dissociation constant of a compound can help us determine the environmental impact of a chemical, e.g., the potential of the chemical to adsorb to soil.
The dissociation constant can also help us to determine the biological impact of a chemical, such as to what degree it will be absorbed by the body and the degree of toxicity. For example, ammonia, NH3, is toxic to fish, whereas its ionized form, Dissociation constants are also useful in helping us to calculate the pH of a weak acid or base.
When an acid, like acetic acid, reacts with water it dissociates into its conjugate base and a single proton which attaches to a water molecule forming hydronium ion.
Some acids, like HCl, are strong acids and dissociate completely, meaning that virtually every HCl reacts with water to produce H3O+1 and Cl-1 ions according to the reaction below. HCl + H2O o H3O+1 Cl-1
Other acids, like acetic acid, are considered weak and only dissociate partially. The degree to which they dissociate is represented by the acid dissociation constant, Ka. The larger the value of Ka, the more the acid dissociates, and vice versa.
The formula for Ka for a generic acid HA reaction with water can be seen below: Ka = ----------------
Notice that the water, a pure liquid, is NOT included in the formula for Ka. Now, applying the pattern above to the reaction of acetic acid with water we get H2O C2H3O2
Ka = -----------------------
>HC2H3O2@
1 6 D E T E R M I N I N G D I S S O C I A T I O N C O N S T A N T S — L A B A C T I V I T Y There are a number of ways that dissociation constants can be calculated. One technique that you may be the most familiar with is the use of a pH meter to determine the dissociation constant of an acid or base (Ka or Kb) in water by conducting a titration. Another method uses a spectrophotometer to determine the concentrations of ionized and unionized forms based on their absorbance spectra. A third way to determine dissociation constants is by measuring the conductivity of a solution at varying concentrations. In this exploration, you will be utilizing this third method.
For a solution to carry a current there must be charged species present that are able to move from one place to another. The measurement of that ability to carry a current is called conductance. Because the species on the left side of the reactions above do not have any charge, they will not contribute to a solution's conductance. The species on the right are all ions and will both contribute to the conductance of a solution. Thus the higher the value of Ka, the more ions are present in solution and the more conductive the solution is. As a result, it is possible to determine the numerical value of Ka by measuring the conductivity of several solutions of differing concentrations. The conductivity probe will be used to measure the conductivity of a series of solutions of both acetic acid and acetylsalicylic acid of varying concentrations. You will convert your conductance measurements to molar conductance, /, by dividing your measurement by the molarity of the solution and dividing by 1000. You will then graph the inverse of molar conductance (1//) vs. concentration · molar conductance (c·/). You will then find the slope of this graph. The value of K = ---------- where m is the
slope of the graph and b is the y-intercept. The mathematical relationship that leads to this graph and analysis is fairly complex, but it is not crucial to understanding the main idea of the lab, that the larger the Ka value of an acid, the more it dissociates and the greater the conductance of the solution.
In this exploration, you will be determining the dissociation constant of two weak acids, acetic acid and acetylsalicylic acid (aspirin), using a conductivity probe. 1 6 D E T E R M I N I N G D I S S O C I A T I O N C O N S T A N T S — L A B A C T I V I T Y Materials
PASCO & Other Equipment
Consumables
Safety Precautions
Follow all directions for using the equipment. Wear safety glasses, gloves, goggles and protective clothing. Be sure to wash your hands if any solutions are spilled onto your skin.
Pre-Lab Question
Which would be a better conductor and why 1.00 M HCl or 0.010M H Could a solution of a weak acid, like acetic acid, ever conduct better than a solution of a strong acid, like hydrochloric acid? Procedure
Equipment Setup
Be sure that all glassware is clean. Rinse all glassware with distilled water to pre-vent ion contamination.
Use a right-angle clamp to hold the Conductivity Electrode to the rod stand. Move the rod stand and Conductivity Electrode over the magnetic stirrer.
1 6 D E T E R M I N I N G D I S S O C I A T I O N C O N S T A N T S — L A B A C T I V I T Y Xplorer GLX Setup
Connect the Conductivity Electrode to the BNC Connector of the Conductivity Sen-sor. Give the connector a twist to lock it into place.
Connect the Conductivity Sensor to Port #1 on the Xplorer GLX. Check that the Conductivity Sensor is set to the middle “Flask” (0-10000 PS/cm) setting.
a Place about 50 - 100 mL of conductivity calibration standard into a 150 mL bea- ker. Add a spin bar and place the beaker on the magnetic stirrer. Begin stirring at a slow to medium rate. Do not allow bubbles to form in the solution, which can happen if the stir rate is too fast.
b Carefully lower the Conductivity Electrode into the solution. The setup should look as depicted in the figure. Adjust the position of the Conductivity Electrode or add more solution until the holes on the side of the Conductivity Electrode are submerged in the solution.
NOTE: Examine the holes on the side of the electrode and check if there are any air bubbles trapped in
the tip of the electrode. If there are air bubbles trapped in the tip, shake or tap the electrode to dislodge
the air bubbles.
Sensors to get to the Sensors Screen.
1 6 D E T E R M I N I N G D I S S O C I A T I O N C O N S T A N T S — L A B A C T I V I T Y e In the Calibration Screen, the Calibration Type should be listed as 1 Point Slope. If it is not, use the Arrow keys to highlight the Calibration Type. Press
f Us e th e Arrow keys to highlight Pt 2 (PS/cm). Press
conductivity value of the standard that you are using. Press g The current sensor reading is displayed on the bottom of the GLX screen. OK to accept the new calibration and exit the calibration screen. and then navigate back to the Digits display.
Record Data
Part 1: Conductivity of Acetic Acid Solutions Rinse all glassware thoroughly using distilled water. Label 3 beakers 0.01 M acetic acid, 0.03 M acetic acid and 0.05M acetic acid.
If your instructor has premixed the different concentrations of acetic acid, add approximately 100 ml of each solution into the appropriately labeled beaker. If not, then use the supplied 0.5 M acetic acid solution and the 100 mL volumetric flask. Add the following amounts of acetic acid into the flask and fill to 100 mL to make these concentrations. Place them in the appropriately labeled beaker: Place a stir bar in the 150-mL beaker containing the 0.01 M acetic acid and put the beaker onto the stirring plate. Begin stirring at a slow to medium rate. Do not allow bubbles to form in the solution, which can happen if the stir rate is too fast.
Rinse the conductivity electrode thoroughly with DI water. Blot the electrode dry with a laboratory wipe. Carefully lower the electrode into the beaker. As mentioned previously, the holes on the side of the electrode must be submerged in the solution. Check for and dislodge any air bubbles trapped in the tip.
to begin recording the conductivity.
1 6 D E T E R M I N I N G D I S S O C I A T I O N C O N S T A N T S — L A B A C T I V I T Y Watch the Digits display and wait for the conductivity to stabilize. Record this value in your table.
Repeat the process for the 0.03 M, then the 0.05M acetic acid solutions. Be sure to thoroughly rinse the conductivity electrode and stir bar with DI water, then blot each dry with laboratory wipes between the samples.
Part 2: Conductivity of Acetylsalicylic Acid (Aspirin) Solutions Thoroughly rinse all glassware using distilled water and blot dry with laboratory wipes. Label 3 beakers 0.01 M acetylsalicylic acid, 0.03 M acetylsalicylic acid and 0.05M acetylsalicylic acid.
If your instructor has premixed the different concentrations of acetylsalicylic acid, add approximately 100 ml of each solution into the appropriately labeled beaker. If not, then use the supplied 0.05M acetylsalicylic acid solution and a 100 mL volumetric flask. Add the following amounts of acetylsalicylic acid into the flask and fill to 100 mL to make these concentrations. Place them in the appropriately labeled beaker: Pour about 100 mL of the 0.05 M acetylsalicylic acid solution into the appropriate beaker.
Repeat the steps from Part 1 above to record the conductances for the 0.01 M, 0.03 M, and 0.05 M acetylsalicylic acid solutions in your table. NOTE: Acetylsalicylic acid solutions are opaque, making it is difficult to view trapped air bubbles in the tip. Assume that they are present and tap the electrode to release any trapped bubbles.
1 6 D E T E R M I N I N G D I S S O C I A T I O N C O N S T A N T S — L A B A C T I V I T Y Data Table
Solution
Conductivity (PS/cm)
Analysis and Synthesis Questions
Using the measured conductivities first convert the value to specific conductivity, N and then molar conductivity, /c, for each of your solutions and enter it into the table below. Remember for our probe specific conductivity = measured conductiv- ity, however, convert PS into ohms-1 (1 PS = 1 x 10-6 ohms-1 ). Molar conductivity = specific conductivity / initial molar concentration.
Solution
Conductance
1 6 D E T E R M I N I N G D I S S O C I A T I O N C O N S T A N T S — L A B A C T I V I T Y Calculate the values for c/ and 1// and enter them in the following table (c = inital molar concentation).
Solution
Plot 1// versus c/ for the acid solutions.
b name the columns cac (for c/) and 1/ac (for 1//).
twice to create two editable columns.
to open the Data Properties window for the first
• In the Data Properties window press
• Use the alphanumeric keys to enter cac as the measurement name then
to select the title of the second column.
to open the Data Properties window. Change the measurement name to 1/a using the alphanumeric keys then press OK.
c Use the Arrow keys to move the Navigating Box to the first row of the cac col-
Edit Cell and enter in the values of c/ for the three differ- ent concentrations of acetic acid by entering a value and then pressing d Use the Arrow keys to move the Navigating Box to the first row of the 1/ac col-
Edit Cell and enter the 1// values for the acetic acid solutions in the same way as the previous values.
1 6 D E T E R M I N I N G D I S S O C I A T I O N C O N S T A N T S — L A B A C T I V I T Y and select 1/ac as the data source for the x axis.
g The plot of 1// versus c/ will be displayed (press to turn on Linear Fit. If there is no data (slope = 0.0), manually select the data points that you entered by pressing the slope and the Y-intercept of the Linear Fit line for your data should now be dis-played. Enter these values into the table.
h Repeat the process for the acetylsalicylic acid solutions by replacing the num- bers in the Table display. Enter the slope and the Y-intercept values into the
table.
Intercept
a = ----------
Calculate the dissociation constants for the two acids and enter them in the table. How do your values compare with published values? Calculated
Published
Dissociation Constant
Dissociation
Constant K
What are some sources of error that could account for the differences between your observed values and the published values? 1 6 D E T E R M I N I N G D I S S O C I A T I O N C O N S T A N T S — L A B A C T I V I T Y

Source: http://www.vrml.k12.la.us/rpautz/documents/Chemistry/DeterminingDissociationConstants.pdf

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