Acid-Base Titration: Calculating the Concentration of an Unknown

Purpose: In this experiment, you will use an acid (HCl) of known concentration to determine the concentration of an unknown solution of NaOH. You will use burets in a process known as a titration. The completion of an acid-base reaction is signaled by an indicator that changes color at the end point. At the end point, you can assume that all the acid has been neutralized and equivalent amounts of acid and base have been used. At this point, the number of moles of acid will be equal to the number of moles of base.

Procedure:

  1. Check to make sure that the burets are clamped securely in their holder. If you are the first group to use the burets, rinse each one out with about 10mL of the solution to be used. Discard any flushed out acid or base.

PLACE THE ACID ON THE LEFT----- PLACE THE BASE ON THE RIGHT

  1. Check to make sure the stopcocks are closed. Fill each buret with the appropriate solution. Record the initial reading in your data table.
  2. Dispense approximately 20mL of acid into a clean, 250mL Erlenmeyer flask.
  3. Add 3-4 drops of the phenolphthalein indicator to the flask.
  4. SLOWLY, add base from the other buret to the flask. Swirl the flask as you titrate. Continue until the pink color stays for a longer period of time. Then, add drop by drop until the end point is reached. The end point will be when the light pink color no longer goes away. If you overshoot the endpoint, add a few drops of acid and then return to the base. Record final readings on both burets.
  5. Repeat the process 2 more times. Be careful not to dispense below the 50mL mark on the buret. Replenish acid and base as needed.

NOTE: You do not have to begin at a reading of 0mL.

Data Table:

Trial 1 / Trial 2 / Trial 3
Initial reading of acid
Final reading of acid
Total volume of acid (Vacid)
Initial reading of base
Final reading of base
Total volume of base (Vbase)
Molarity of base

Calculations: Calculate the concentration of HCl in all three trials and then average them together. Round each concentration to 3 decimal places.

Post Lab Questions:

1.) 86.30 mL of an HCl solution was required to neutralize 31.75 mL of 0.150 M NaOH.

Determine the molarity of the HCl.

2.) 63.15 mL of calcium hydroxide is required to titrate 18.9 mL of a 0.200 M H3PO4 solution. What is the molarity of the basic solution? (Note: They are not monoprotic…You must do the solution stoichiometry steps.)

Conclusion: