Acid Lab
Purpose: 1. Titrate two brands of vinegar with 1 M solution of NaOH to determine the
molarity of the vinegar.
2. Use the molarity of the vinegar to determine the pH of the vinegar.
Facts you need to answer question #1 READ THIS BEFORE ASKING ME QUESTIONS: Acetic Acid is CH3COOH and vinegar is simply acetic acid in water. Vinegar is a 5% solution of acetic acid in 95% water. This percentage is by mass. So, in 100 g of vinegar, 5 g of that is acetic acid and 95 g is water. Vinegar has the same density as water and so 100 mL of vinegar is also 100 g.
1. Assume you have 10 mL of vinegar. How many grams of acetic acid are in the vinegar?
2. How many moles of acetic acid are in the 10 mL sample of vinegar?
3. Find the molarity of Acetic Acid (CH3COOH) in your 10 mL sample of vinegar. Record this on your data table as the Molarity of Acetic Acid based on pre-experiment calculations
4. Write the balanced equation of acetic acid in water (remember it is in equilibrium). Also remember that all acid/base reactions have coefficients of ONE. Also remember to put in (l) and (aq) where necessary. Also remember to put in + and – charges when necessary.
5. Write the Ka (using symbols).
Then, Find the theoretical pH of vinegar by using:
1) The Ka you just wrote
2) The theoretical molarity you found in question #3 and
3) The Ka for acetic acid which is 1.8 x 10-5 .
Once you have found the theoretical pH. Record this as THEORETICAL pH on your data table.
6. Write the balanced equation of acetic acid being mixed with NaOH. (remember it is in equilibrium). Also remember that all acid/base reactions have coefficients of ONE. Also remember to put in (l) and (aq) where necessary. Also, an Acid + a Base = a Salt + a Water
7. How many moles of NaOH are going to be required to neutralize the moles of acetic acid you determined in question #2? Hint: Look at the coefficients to help you determine the correct mole ratio. This is SUPER SIMPLE!
8. Assume you have a 1.00 M solution of NaOH. How many mL of this solution will it take to make the number of moles you determined you were going to need in question #7? Record this as Predicted volume of NaOH to be titrated in your data table.
THIS IS VERY IMPORTANT: Your answer to #8 is a good approximation of
how many mL of 1.00 M NaOH you will be titrating into your 10.0 mL sample of
vinegar.
Procedure:
BIG HINTS: 1) Use a white 3x5 card to read the buret and another one underneath the Erlenmeyer flask to see the color change. 2) Read the buret from the bottom of the meniscus. 3) Always have a “waste” beaker handy to catch any liquid you need to drain out of the buret. 4) Never, Never, Never mix plastic pipettes. If you see a plastic pipette in one solution, NEVER, NEVER, NEVER remove it and place it in another solution! 5) As you titrate, use the magnetic stir bar to keep the solution stirred.
1. Fill your buret with 50.0 mL of 1.00 M NaOH. Here’s a hint: Fill the buret to ABOVE the 0 mL line and then let it drain down to the 0 mL line. Always drain into your “waste” beaker.
2. Use the plastic pipette in the beaker of Heinz Vinegar and put 10.0 mL of vinegar into a graduated cylinder and then transfer it to a 250 mL Erlenmeyer flask. (PUT A STIR BAR IN THE FLASK!)
3. Take the beaker of vinegar over to a
computer which is hooked up to a pH meter.
Record this as Actual pH from pH meter in
your data table.
4. Next, add 2 drops of Phenolphthalein to the 250 mL Erlenmeyer flask. NO MORE THAN 2 Drops!
5. Begin to titrate the 1.00 M NaOH into the Erlenmeyer flask. Remember to put a white 3x5 card under the Erlenmeyer and use another white 3x5 card to read the buret. When the Erlenmeyer remains “just a light hint of pink”, you are done. The trick is the Erlenmeyer is supposed to REMAIN a light hint of pink and not turn back to clear. When you are done titrating, record the volume of 1.00 M NaOH you titrated as Actual Volume of 1.00 M NaOH titrated to the flask on your data table
What happens if you overshoot? If you overshoot the endpoint, come back and get
5 mL more of vinegar and add it to your Erlenmeyer flask and continue to titrate. Remember to increase the amount of vinegar you had to start with by 5 mL
7. Repeat the experiment with Albertson’s Vinegar.
Calculations:
Determine the Molarity of Acetic Acid based on experiment. (see the line for this
on your data table below) Show all of
your work for both the Heinz Vinegar and the Albertson’s Vinegar.
Data Table:
Volume of vinegar in Erlenmeyer flask ______10_______mL _________L
Molarity of Acetic Acid based on experiment ___________M*(see hint below – be sure to show calculations for this step)
Molarity of Acetic Acid based on pre-experiment calculations (pre-lab question #3) _________M
Difference between experimental molarity and pre-calculation molarity ___________
Percentage Error = (|actual value – expected value|/ expected value) x 100 ______________%
Predicted volume of NaOH to be titrated ___________mL ________L
Actual Volume of 1.00 M NaOH titrated to the flask ___________mL ________L
THEORETICAL pH (pre-lab question
#5) ___________________
Actual pH from pH meter (lab procedure #3) ________________
HINT* Multiply the volume of NaOH you added (in Liters) by the Molarity of NaOH to determine the number of moles of NaOH you added. Remember that the number of moles of NaOH you added is equal to the number of moles of CH3COOH in the Erlenmeyer (if you don’t believe this, look at the balanced equation from pre-lab question #6). To find the molarity of CH3COOH, simply divide its moles by its total original volume (in Liters).
Albertson’s Vinegar
Volume of vinegar in Erlenmeyer flask _____10________mL ____________L
Molarity of Acetic Acid based on experiment ___________M
Molarity of Acetic Acid based on pre-experiment calculations (pre-lab question #3) _________M
Difference between experimental molarity and pre-calculation molarity ___________
Percentage Error = (|actual value – expected value|/ expected value) x 100 ______________%
Predicted volume of NaOH to be titrated ___________mL ________L
Actual Volume of 1.00 M NaOH titrated to the flask ___________mL ________L
THEORETICAL pH (pre-lab question
#5) ___________________
Actual pH from pH meter (lab procedure #3) ________________
Questions:
1. Is acetic acid a weak or a strong acid? (Remember your weak/strong acids and bases for the exam!)
2. Finish this statement: The smaller the Ka, the _________ (lower or higher) the pH
3. Find the pH of the following molarities of HCl (remember it is a strong acid).
a. 1.0 x 10-3 M b. 1.0 x 10-5 M c. 2.6 x 10-3 M d. 3.1 x 10-5 M
4. Find the molarity of the following pH values for HCl (remember it is a strong acid).
a. 2 b. 4 c. 3.15 d. 2.91
5. Find the pH of the following molarities of acetic acid (remember it’s a weak acid).
a. 1.0 x 10-3 M b. 1.0 x 10-5 M c. 2.6 x 10-3 M d. 3.1 x 10-5 M
6. Find the molarity of the following pH values for acetic acid (remember it’s a weak acid).
a. 2 b. 4 c. 3.15 d. 2.91