NAME

DATE

Chemistry 1°

Lab C33: Determine pKa by Half Titration

 

OBJECTIVES

 

INTRODUCTION

Acids in water can ionize water, forming the hydrogen (H+) or hydronium ion (H3O+) and the degree of ionization determines the strength of the acid. When ionization is complete, there is no molecular acid in the solution. Complete ionization indicates a very strong acid.

The rate constant known as kf (forward reaction rate) informs of the amount of dissociation of an acid. Stronger acids are so large that there is no reverse reaction, meaning that kr is equal to zero. Weak acids will ionize but the ions return to their original molecular form. Thus, they continue ionizing and going back until equilibrium is reached, meaning the rate of formation is equal to the rate of ionization. The ratio of kf to kr (also known as Ka, or the equilibrium constant) is less than one for a weak acid and is in the following form: Ka= ([H+]*[X-])/[HX]. The pKa of an acid is the negative log of Ka. A large pKa indicates a small Ka and a less ionized weak acid.

A base and an acid combine to form salt and water. The salt formed by a weak acid is called a buffer and it will not exhibit large changes of pH since it provides an acid source and a base source. Though it will not ionize proficiently, the sodium salt of a weak acid can react with an acid and the pH of the solution will equal the pKa of the weak acid, following the equation: pH = pKa + log [salt]/[acid]. Using this equation, the pH can be determined at any point of the titration.

 

MATERIALS

base stand and support rod

magnetic stirrer

50-mL buret

2 250-mL beakers

1 buret clamp

pH calculating device

1 spin bar

50-mL 0.10 molar NaOH solution

100-mL 0.1 molar acetic acid

100-mL high pH buffer solution

100-mL low pH buffer solution

funnel

graduated cylinder

wash bottle

SET-UP

Safety

  1. Wear protective gear.
  2. Follow directions for using the equipment.
  3. Handle and dispose of all chemicals and solutions properly.

Instructions

  1. Place 100 mL of acetic acid in a 250-mL beaker. Put a spin bar in the beaker.
  2. Place the beaker with acid and spin bar on the magnetic stirrer.
  3. Use the buret clamp and base and support rod to position the pH calculating device so the end of the device is in the acid, but will not interfere with the spin bar.
  4. Use another clamp to support the 50-mL buret so the end of the buret is above the acetic acid. Make sure the buret valve is closed.
  5. Put 50 mL of the NaOH solution into the buret.
  6. Turn on the magnetic stirrer.

 

 

PROCEDURES
  1. Take the pH before adding any of the hydroxide solution.
  2. When ready, open the buret valve to allow the solution to allow 1 mL of the sodium hydroxide in. Take the pH level and record in Data Table 1. Repeat until all the sodium hydroxide is gone.
  3. Once the sodium hydroxide is added to the acid, stop recording the data and turn off the magnetic stirrer.
  4. Record the ending pH level in Data Table 1.
  5. Remove the pH electrode with caution. Make sure to rinse well with distilled water and put it in its bottle of buffer solution.
  6. Dispose of the mixture.

    7.  

    RESULTS

    [Graphs attached]

     

    Data Table 1

    Item Value

    Beginning pH 2.5

    Ending pH 4.8

    CONCLUSIONS

    What do you think?

    How does the strength of an acid as measured by its pKa affect the shape of the acid’s titration curve?

    A steep titration curve signifies a strong acid. So the steeper the curve, the stronger the acid.

    Questions

        1. What was the pH of the starting acetic acid solution?

The pH of the starting acetic acid solution was 2.9.

2. What happened to the pH of the solution as the NaOH was added? Is this what you expected to happen?

The pH level of the solution rose as the NaOH was added. Yes, we expected this to happen since NaOH is a base. Also, the values leveled off when they reached equilibrium. Since we used the wrong concentration of NaOH, .3 mol instead of .1 mol, we had to look much earlier in the graph to see the leveling off. In our graph, after reaching equilibrium, the graph became even more basic because of such a high level of concentration of NaOH.

  1. The pH at the end of the plot gives the pKa of the acid. What is the pKa of the acetic acid?

pH = pKa + log [salt] / [acid]

4.8 = pKa + log [.01]/[.01]

4.8 = pKa

4. The negative antilog of the pKa is the Ka of the acid. What is the Ka of the acid?

pKa= - log (Ka) = 4.8

Ka = 10^-4.8

Ka = 1.5 x 10^-5

5. Sketch the titration graph for a similar titration with a different acid that has a pKa equal to 2.5. Is this acid a weak or strong acid?

This is a strong acid since the pKa is lower.

6. The acid used in this experiment is a monoprotic acid which means only one hydrogen ion is ionized. What would happen to the titration graph if a diprotic (two hydrogen ions are ionized) acid would replace the acetic acid used in this experiment?

A monoprotic acid only ionizes one hydrogen ion but a diprotic one ionizes two hydrogens. Thus, the titration graph of a diprotic would be different from that of a monoprotic. A diprotic titration graph will have two peaks reached, whereas monoprotic only have one. There will be the large rise of pH and the leveling off for the first ionized hydrogen and then a second rise, much shorter. Then, it would also level off.