Designing a Buffer
Goals:
1. To determine the Ka for the experimental conditions present in the laboratory
2. To make a buffer of a desired pH
3. To predict by how much the pH will change when acid or base is added to a buffer
This experiment involves the acetic acid/sodium acetate buffer system. You will manipulate the equilibrium to achieve a desired pH. Then, you will add acid or base to the buffer to further change the equilibrium.
CH3COOH + H2O ⇆ CH3COO- + H3O+
You will need to use all of your knowledge of buffers and buffer calculations to combine the weak acid and the salt in the correct combination to match the assigned pH.
The first part of the experiment allows you to determine the actual Ka for acetic acid. The Ka of a weak acid depends on the concentration and the temperature of the solutions, in addition to some other factors. The Ka we have been using in class for acetic acid, 1.8 x 10-5, would enable us to make solutions close to the desired pH. However, if we have a Ka that more accurately reflects the experimental conditions, we will be able to make buffers much close to the desired pH.
In Henderson-Hasselbalch’s equation, if the concentration of weak acid and salt are the same, then the logarithmic term is zero and the pH is equal to the pKa. Using this shortcut, chemists can easily determine more accurate Ka’s for the concentrations and temperatures present in the laboratory.
pH = pKa + log ([base]/[acid])
pH = pKa + log (0.10/0.10)
pH = pKa + log 1
pH = pKa
In the second part of the experiment, you will create a buffer at your assigned pH. The teaching assistant will check to see how close you came to the assigned pH. If you are off, you have the option of re-doing this part of the experiment with a corresponding grade penalty. However, since the third part of the experiment depends on how good a job you’ve done in making your original buffer, if you are more than 10% off, you would be wise to re-do the second part of the experiment before doing the next part of the laboratory.
In the third part of the experiment, you will add a known amount of acid to one-half of the buffer solution and predict what the new pH will be. To the other half of the buffer solution, you will add a known amount of base and will again predict what the new pH will be. The teaching assistant will check to see how close your prediction is to the actual pH.
· YOU WILL WORK ALONE ON THIS EXPERIMENT.
· THE TEACHING ASSISTANT AND LABORATORY INSTRUCTOR CANNOT HELP YOU DO YOUR CALCULATIONS OR MAKE DECISIONS.
· YOU MAY NOT CONSULT YOUR NEIGHBORS.
· YOU MAY USE ANY NOTES OR TEXT YOU NEED TO COMPLETE THE WORK.
Part One: Determining the Ka
You need to determine the pH under conditions of concentration and temperature that are as close as possible to the conditions you will be using in laboratory today.
The acetic acid solutions in the laboratory are about 3M. You will be diluting the stock solution of acetic acid to concentrations of around 0.12 M. We will make 50 mL samples of the buffer. Determine roughly how much stock solution of acetic acid is required to make a 50 mL solution of about 0.12 M concentration. Round up or down to get to a volume of stock solution that can be measured using the volumetric pipets in the laboratory.
Pour a small amount of acetic acid stock solution into a clean, dry beaker. The amount you put into the beaker should be only slightly more than you need for your pipet so that there will be little wasted chemical. Never pipet directly from the stock bottles. Use volumetric pipets available in the laboratory to transfer the desired amount of acid to a 50 mL Erlenmeyer Flask. Do NOT dilute yet.
Based on the concentration of acid you will have after you dilute to 50 mL, determine how many grams of sodium acetate you will need to add to the dilute acid so that the concentrations of both the weak acid and the salt are identical. Weigh out that amount of sodium acetate at the balance and carefully transfer the entire amount of salt to the volumetric flask. Add water to the 50.00 mL mark and mix well.
Take your solution to the TA to determine the pH of the buffer. From the pH, determine the actual Ka for acetic acid solutions under the conditions of concentration and temperature present in today’s laboratory. Use this Ka throughout the entire experiment for best results. Be aware of significant figures as you find the Ka.
Part Two: Making the Buffer
In part one, the ratio of salt to acid was 1:1. For the second part of this experiment, we will use the same concentration of dilute acetic acid we used in part one (0.12 M roughly), but alter the amount of salt to obtain the pH you were assigned.
Using the Ka you found in part one, determine how many grams of sodium acetate you need to add to the acetic acid to obtain your assigned pH. Make a 50.00 mL sample of the buffer (roughly 0.12 M acetic acid plus the salt) and mix it well.
Bring your volumetric flask to the teaching assistant to check the pH. The teaching assistant will remove a small portion of the buffer, check the pH and then discard that portion of the buffer. Retain the rest of your buffer solution for the third part of the experiment.
Part Three: Adding Acid or Base to a Buffer
Pour about half of the remaining buffer solution into a clean and dry graduated cylinder. Note the exact amount of buffer in the graduated cylinder.
Pour slightly more than 5 mL of dilute HCl into a clean, dry beaker. Using a 5.00 mL volumetric pipet, added dilute HCl to the buffer. Use a dry stirring rod to thoroughly mix the acid into the buffer. Calculate the expected pH and then bring the graduated cylinder to the teaching assistant. The teaching assistant will use a pH meter to see how close the actual pH is to the predicted pH. Dispose of this portion of the buffer/acid mixture after the teaching assistant has determined its pH.
Rinse and dry the graduated cylinder. Pour the remaining buffer into the cylinder and note the exact amount of buffer that was transferred.
Pour slightly more than 5 mL of dilute NaOH into a clean, dry beaker. Using a 5.00 mL volumetric pipet, added dilute NaOH to the buffer. Use a dry stirring rod to thoroughly mix the base into the buffer. Calculate the expected pH and then bring the graduated cylinder to the teaching assistant. The teaching assistant will use a pH meter to see how close the actual pH is to the predicted pH. Dispose of this portion of the buffer/base mixture after the teaching assistant has determined its pH.