Lesson

Background

Titration is the name given to the process for determining the volume of solution needed to react with a given volume (or mass) of a sample. You will use this process to study quantitatively the reaction between an acid and a base. A common neutralization reaction is the one between the hydrogen ion of an acid and the hydroxide ion of a base to form water:

Acid + Base ⇾ Salt + Water

An indicator solution is used to determine when an acid has exactly neutralized a base. A suitable indicator changes color when equivalent moles of hydrogen ions and hydroxide ions have reacted, this is termed the equivalence point of the titration. That is, [H⁺] = [OH^-] at the equivalence point.

Purpose

Your job in this lab is to correctly determine the molarity (concentration) of HCl by titrating it with sodium hydroxide (NaOH) of known concentration. Then, you will calculate the reaction rate

Materials

• Safety glasses
• Lab apron
• Unknown [HCl]
• Known [NaOH]
• Phenolphthalein
• Burette
• 500-mL volumetric flask
• 250-mL beaker
• Funnel

Safety

• Always wear safety goggles and a lab apron to protect your eyes and clothing.
• If you get a chemical in your eyes, immediately flush the chemical out at the eyewash station while alerting the teacher.
• Do not touch any chemicals with your bare hands.If you get a chemical on your skin or clothing, wash the chemical off at the sink while alerting the teacher. Make sure you carefully read the labels and follow the precautions on all containers of chemicals that you use.
• Do not taste any chemicals or items used in the laboratory.
• Never return leftovers to their original container; take only small amounts to avoid wasting supplies.

Prelab Questions

1. What type of reaction will be taking place in this titration lab?
2. What is meant by the term “equivalence point”?
3. Write the balanced equation for the reaction in this titration. Identify the acid, base and salt.
4. You will be adding the indicator phenolphthalein to the HCl in your flask.
   a. What color will the solution in your flask be before you start adding base?
   b. What color will the solution turn when you have added enough base?
5. Record the volume of each of the burettes below to the correct number of significant figures.

Procedure

1. Obtain about 100-mL NaOH solution. Check the label to see what concentration the NaOH solution is. Record this concentration in your Data Table.
2. Obtain a burette and prepare it for use. This involves the following procedures:
   1. Close the stopcock (valve at the bottom of burette).
   2. Rinse the burette with the NaOH solution.
   3. Fill the burette with the NaOH solution you prepared.
   4. Fill the tip of the burette by allowing several milliliters of solution to run from the burette into a waste beaker. This removes air bubbles.
   5. Read the starting volume graduation (record this starting volume of your Data Table) on the side of the burette. Notice that the burette volumes increase as you progress DOWN the burette.
3. Using a clean, dry graduated cylinder, measure 40 mL of your HCl solution (of unknown concentration) and pour into a clean, dry 250-mL Erlenmeyer flask.
4. Add two-three drops of phenolphthalein indicator to your acid solution.
5. Deliver about one mL of the base solution from your burette into the flask containing the acid and indicator.
6. Swirl the container and add another one mL of base.
7. As you continue the adding and swirling, you will notice that a pink color appears in the solution and then disappears after swirling. Finally after several repetitions of this process the pink color will remain. Read and record in your Data Table the burette volume at this equivalence point.
8. Subtract your "reading before titration" from your "reading after titration" to obtain the approximate volume of base used. You have now completed your "practice or approximation" trial.
9. Empty the contents of your flask into the sink. Rinse the container with water.
10. Repeat your titration. Since you have the volume required in your "practice" trial, you can quickly titrate to within about three-four mL of that volume. Now add the base more slowly - a drop per second is a desirable rate - with constant swirling. As the color begins to persist, slow the rate to a drop every 10-15 seconds. Finally as you are very close to the equivalence point, add the base one drop at a time, opening and closing the valve for each drop. An exact equivalence point (your goal) occurs when the solution is still colorless, and 1 additional drop causes the solution to turn a FAINT pink when swirled and then remain pink for at least 30 seconds.
11. Do at least three trials where you stop at the exact equivalence point. With good technique and careful work, you can obtain 3 trials in which the variation in volume of base used is 0.5 mL or less.
12. Cleanup Procedure: To clean the burette, drain all remaining NaOH solution from the burette. Complete two water rinses by closing the stop cock and filling the burette with tap water, and then draining this water. Complete a water rinse with about 20 mL of distilled water and the stop cock open.

Results/Observations

Calculations

1. Calculate the volume change of base in each trial. (This should be done in the right-most column in your data table above.
2. Given that you know the acid volume, the base concentration, and the volume change of the base for each trial, calculate the molarity of the acid using the titration equation M_AV_A= M_BV_B. Show all work.

3. Average each of the molarities you calculated for in problem 2. This is your experimental value.
4. Figure out the accepted value for the concentration of acid by calculating a dilution. The HCl label says 76 mL of 12-M HCl was diluted to 1 L.
5. Calculate your percent error. ((accepted value – experimental value) / accepted value ) x 100 %
6. What volume of 6.0-M HCl is needed to neutralize 2.0 L of 0.10-M NaOH? Show all work.
7. Describe why you wanted the solution to be a pale pink color when the titration was over rather than a dark pink. Hint: When does phenolphthalein turn pink? How does that range relate to the equivalence point?
8. When a substance is considered to be neutral, what is its pH? ______________
9. Now that you know the concentrations of your reactants in the neutralization equation, calculate the reaction rate. First, write the rate law for the reaction if both reactants are first order. Then, calculate the rate using your experimental HCl concentration and the given NaOH concentration. (Let’s say k=1.2 x 10^-5 s^-1)
10. What is the overall reaction order in this rate law? ______________