Determining Ka by the Half PDF

Preview

Summary

ccc...

Description

CHEM 1102_07_Sp_20

Joshua Thompson

Tuesday, March 24, 2020

Determining Ka by the Half-Titration of a Weak Acid Abstract: In the experiment we did an acid-base titration with an acid and NaOH. With my experiment I found the pKa to be 3.84. To reach the equivalence point in trial 1 we added 14.25 mL of 1.5 M solution. In trail 2 we added 28.25 mL of 0.75 M solution. In trail 3 we added 47.25 mL of 0.45 M solution. Introduction: A common analysis of a weak acid or a weak base is to conduct a titration with a base or acid of known molar concentration to help determine the equilibrium constant, Ka, for the weak acid or weak base. If this titration is conducted very carefully and very precisely, the results can lead to a valid approximation of an equilibrium constant. In this experiment the half-titration point will exist when you have added half as many moles of HA as moles of NaOH. Thus, OH–will have reacted with half of HA, leaving the solution with equal moles of HA and A–. At this point, according to the Henderson-Hasselbalch equation, pH = pKa + log [A-]/[HA] if there are equal moles of HA and A– at the half-titration point, then pKa is equal to the pH value of the solution. Materials/Procedures: All procedures where followed correctly Analysis/Discussion/Conclusion:

Volume of acid (mL) Molarity NaOH (M) Volume of NaOH (mL) Volume at Equivalence Point (mL) Volume at HalfEquivalence point (mL) pKa of unknown acid

Citation:

Trail 1 25.0 1.5 20

Trail 2 25.0 .75 35

Trail 3 25.0 0.45 50

14.25

28.25

47.25

7.125

14.125

23.625

3.84

3.84

3.84

CHEM 1102_07_Sp_20

Joshua Thompson

Tuesday, March 24, 2020

1. Explain why the pH at the half-titration point is equal to the pKa in your experiment. At the half-equivalence exactly half the acid has been neutralized so pH = pKa + log [A-]/[HA] a. 2. Does changing the concentration of NaOH have any influence on the pKa of the acid. Explain your answer. a. The concentration doesn’t affect the Pka of the acid. With a higher concentration the equivalence point is simple reached with less mL of NaOH but, doesn’t change the calculated Pka. 3. Explain how this test could be done using only an indicator solution and no electronic means of measuring PH. a. You can use the various indicators to identify the ph. When the color changes that means Equivalence, point was meet

Silberberg, Martin S., and Particia G. Amateis. Chemistry: The Molecular Nature of Matter and Change. 8th ed., McGraw-Hill Higher Education, 2018. Department of Chemistry. 2019. Determining Ka by the Half-Titration of a Weak Acid: Experiment 5. CHEM 1102_07_Sp_20 Laboratory Handout. Texas. University of the Incarnate Word....