Unit 7: Acid-Base Chemistry

Content Outline: Performing Titrations & Uses (7.8)

  1. Titration (“titrate” means to “measure out”)
  1. The controlled addition and measurement of the amount of a solution having a known concentration of acid or base required to react completely with a measured amount of a solution having an unknown concentration.

In plain English, we are using a known concentration solution to find the concentration (pH) of an unknown solution.

  1. This process involves a neutralization reactionoccurring.
  1. This can be viewed as either:
  1. H3O+(aq) + OH-(aq) 2H2O(l)
  2. Strong Acid + Strong Base  Salt + Water
  1. Equivalence Point This is the point during a titration process at which the two solutions concentrations used are present in equivalent amounts.
  1. End Point
  1. The point during a titration at which the acid-base indicator (that was added to the unknown sample) changes color.
  1. The color must appear throughout the solution and persist for more than 30 seconds while stirring.
  1. Standard Solution

A. This is the solution with the known concentration of solute.

  1. Correct Procedure for performing a Titration:
  1. Materials required:
  1. Ring Stand with Double Biuret clamp attached.
  2. 2 clean 50mL Biurets
  3. Clean Erlenmeyer flask
  4. Acid-Base Indicator solution
  5. Known concentration solution (acid or base)
  6. Unknown concentration solution
  1. Setup Procedure:

Step 1: Place the Biuret clamp onto the Ring Stand.

Step 2: Place your empty Erlenmeyer flask on the lab bench next to the stand and under the clamp.

Step 3: Position each Biuret, in the clamp, so that you can easily slide your Erlenmeyer flask under the Biuret dispensing end without hitting the Biuret. The tip of the Biuret should fit just slightly into the flask. You may need to raise or lower the Biuret, not the clamp.

Step 4: Take a piece of paper and write “known” on it. Place the paper under the Biuret you will use to hold your known concentration solution.

Step 5: Take a piece of paper and write “Unknown” on it. Place the paper under the Biuret you will use to hold your unknown concentration solution.

Step 6: Place the empty Erlenmeyer flask under the “Known” Biuret.

Step 7: Make sure your Biuret stopper is open (running up/down). Using a clean funnel inserted

into the top of the Known Biuret, rinse it 3 times with the “Known” solution. Be sure to get the entire Biuret “wet” on the inside only. This is to remove any possible contaminants in the Biuret.

Step 8: Rinse your funnel 3 times with distilled water and dry.

Step 9: Empty the “Known” solution out of the Erlenmeyer flask and rinse.

Step 10: Place the rinsed Erlenmeyer flask under the “Unknown” Biuret.

Step 11: Make sure your Biuret stopper is open (running up/down). Using the same clean funnel

inserted into the top of the Unknown Biuret, rinse it 3 times with the “Unknown” solution. Be sure to get the entire Biuret “wet” on the inside only. Thisis to remove any possible contaminants in this Biuret as well.

Step 12: Empty the “Unknown” solution out of the Erlenmeyer flask and rinse.

Step 13: Rinse your funnel 3 times with distilled water and dry.

Step 14: Close the stopper (horizontal) on both Biurets.

Step 15: Use the clean funnel and gently fill the “Known” Biuret with the “known solution” to slightly above the “0mL’ markand make sure there are no bubbles in the Biuret.

Step 16: Take the empty Erlenmeyer flask and place under the “Known” filled Biuret.

Step 17: Using the stopper, slowly release the excess fluid out until the meniscus is at 0mL and close the stopper. This will also get rid of unwanted air bubbles.

Step 18: Empty the “Known” solution out of the Erlenmeyer flask and rinse.

Step 19: Rinse your funnel 3 times with distilled water and dry.

Step 20: Use the clean funnel and gently fill the “Unknown” Biuret with the “known solution to

slightly above the “0mL’ mark and make sure there are no bubbles in the Biuret.

Step 21: Take the empty Erlenmeyer flask and place under the “Unknown” filled Biuret.

Step 22: Using the stopper, slowly release the excess fluid out until the meniscus is at 0mL and close the stopper. This will also get rid of unwanted air bubbles.

Step 23: Empty the “Unknown” solution out of the Erlenmeyer flask and rinse 3 times with distilled water. Then dry.

  1. Performing the titration:

Step 1:Record your starting fluid level in both Biurets and the initial color of each solution.

Step 2: Place your clean dry Erlenmeyer flask under the “unknown” Biuret.

Step 3: Using the stopper, slowly measure out the required amount of “unknown” solution as directedby your teacher.

Step 4: Record the amount of unknown released.

Step 5: Remove the flask from under the Biuret. Add to the flask, 3 drops (or amount direct by your teacher) of the acid-base indicator to the Erlenmeyer flask. Then gently stir to get equal distribution. Record the color of the solution.

Step 6: Place the Erlenmeyer flask under the “known” Biuret.

Step 7: Slowly release, while gently swirling the flask, the known by rotating clockwise the stopper 1 half turn. You will see a slight color change, but it will quickly change back as you swirl.

Step 8: Repeat step 7 until the color light change persists for more than 30 seconds. You have

reached the end point.

Step 9: Record the final volume you added to the flask by looking at the meniscus of your “Known

Biuret. That number is the delivered volume.You started at 0.

Step 10: Record the final color of the solution.

Step 11: Empty the solution out of the Erlenmeyer flask and rinse 3 times with distilled water. Then dry.

Step 12: Repeat Steps 1 – 11 two more times. This will give us three sets of data to work with.

  1. Determining the concentration of the Unknown from our Data Start by making a balanced complete neutralization chemical reaction involving the Known solution and Unknown solution. One should be an Acid and one a base. Do not include the indicator.
  2. Determine the number of moles used (added) to the unknown solution.

Concentration of Known X 1 Liter X Quantity added in mL = Number moles of Known

1 Liter 1000 mL

  1. Adjust the mole to mole ratio, if not a 1:1 ratio

Number moles of Known from above X moles of Unknown (equation) = Adjusted mole value

Moles of Known (equation)

  1. Calculate the concentration (Molarity) of the Unknown solution:

Mole Value of known X 1000 mL = Concentration of Unknown

Quantity of Unknown measured out in mL 1 Liter

Remember, Molarity = mol/L