Weak Acid/Base Equilibria Calculations and Measurements
Wear safety glasses at all times while working with these acidic and basic solutions.
Use of the pH meters:
There are pH meters at several lab stations throughout the lab. The teaching assistant will be present to show you the details of the meter. Bring the solution you need to measure to the pH meter in a clean dry 100 mL beaker. If you need to make a series of measurements for a particular exercise, be sure to use the same pH meter for each measurement in that exercise, to eliminate systematic error.
Percent Dissociation of Acetic Acid
We have seen in class that percent dissociation of an acid decreases as the acid concentration increases. Percent dissociation is measured as:
You will investigate this for acetic acid (H3CCO2H). Take from the reagent hood 50 mL of 2 M acetic acid in your large graduated cylinder. Pour the 2 M acetic acid into your clean dry pH measuring beaker. Select a pH meter and measure the pH of your 2 M acetic acid solution with the help of your TA.
Convert the measured solution pH to [H+]. Use the measured [H+] todetermine the % dissociation of acetic acid at a concentration of 2 molar.
Next you will begin a series of dilutions, pH measurementsand calculations of % dissociation.
Measure 5 mL of the 2M acetic acid with your small graduated cylinder. Add the 5 mL of 2M acetic acid to your large graduated cylinder. Carefully fill the graduated cylinder to the 50 mL mark with DI water. You will want to add the last few milliliters of water with a squirt bottle to improve precision. Calculate the initial acetic acid concentration in thisdiluted solution.
Empty the remaining 2M acetic acid in the pH measuring beaker into the acid waste container at the front of the room. Rinse and dry the measuring beaker so that the second pH measurement can be made. In order to be sure that the diluted acid is completely mixed, pour from the graduated cylinder into the pH beaker and back to the graduated cylinder several times. After two or three mixings, make a pH measurement of the diluted solution.
Use the diluted acidto make a second dilution that yields 50 mL of a 0.04 M acetic acid solution. After making the dilution in your graduated cylinder, make a pH measurement of the new solution. Be sure that you start with a clean dry beaker for measuring pH and completely mix the solution by pouring between the two containers.
Make a third dilution that results in 50 mL of 0.004 M acetic acid. After preparing the solution, record the solution pHin the manner previously used.
Make a final dilution that results in 100 mL of 0.001M acetic acid. After preparing the solution, record the solution pH in the manner previously used.
From the measurements of the five acetic acid solutions make a table in your lab notebook that reports: Acid Initial Molarity ([HAc]0), pH, [H+], and % dissociation.
Briefly describe in a short paragraph the trend in the relationship between:
pH, [H+] and % dissociation relate to initial acid concentration, [HAc]0.
Determining the Ka of Mandelic Acid
Mandelic acid is a weak organic acid (MM = 152 g/mol). The structure of the acid is given in Figure 1.
Figure 1. Molecular structure of mandelic acid
Sketch this structure into your lab notebook. In your molecular drawing show all the carbons and the hydrogens. Be sure that your structure has the correct molecular mass.
In your drawing circle all the atoms you expect to have sp3 hybridization.
There are two hydrogen atoms in the molecule that are the likely source of the acidic proton. It turns out that in water only one of these protons dissociate. Draw the molecular structure for the conjugate base of mandelic acid (the mandelate ion).
In a 100 mL volumetric flask (available at the front of the room) prepare an approximately 0.05 M solution of mandelic acid. As you prepare this solution record all mass measurements such that you can calculate the actual concentration of your solution to three significant digits.
As you use the balance to weigh out the required mandelic acid into a weigh boat, be sure to never add the reagent to the weigh boat while it is on the balance.
As you transfer the mandelic acid to the volumetric flask be sure that it all is transferred. You should use a squirt bottle to rinse your weigh boat into your volumetric flask.
After analytically transferring all of your mandelic acid to the volumetric flask, fill the flask 2/3 full with DI water. Cover the top of the flask with parafilm and shake to dissolve the acid. Complete the final addition of water to the volume mark with a squirt bottle.
After preparing the solution, measure the pH of your mandelic acid solution using a pH meter.
In your lab notebook write the I.C.E. dissociation table for mandelic acid (HMA) dissociation. X in the I.C.E. table is related to the solution pH. From the initial concentration of your acid and the measured pH, determine the Ka of this acid.
All of the mandelic acid solution can be placed in the acid waste beaker after your measurements have been taken.
After completing all of the pH measurements and calculations for this lab, clean and put away the glassware you used. Turn in the duplicate sheets from your laboratory notebook. There is not an abstract due with this laboratory work.