CHEM1831 Lecture Notes - Lecture 18: Hexane, Substitution Reaction, Alkene

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9 Aug 2018

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For any aqueous solution at 25 degrees: ph + poh = 14. Naturally occuring buffers control the ph in biological fluids as blood (one aspect of homeostasis). In blood the dominant equilibrium which controls ph is: H2co3 hco3^- (aq) + h+ (aq) Buffer - has to have a weak acid and a congruent base with the weak acid. A titration involves adding a solution accurately known concentration (standard solution) to a solution of unknown concentration until the reaction between the two solutions is complete. The equivalence point can be detected with an indicator. An acid base indicator is a weak acid or base. The end point of a titration is indicated by the change in colour of the indicator. By using a suitable indicator, the end point will occur very close to the equivalence point. Strong acid, strong base - can use most types of buffers.

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Consider a weak acid-strong base titration in which 25 mL of 0.120 M of acetic acid is titrated with 0.120 M of NaOH.

a) Calculate the pH of the acetic acid solution BEFORE addition of NaOH (pKa of acetic acid = 4.75).

b) Calculate the pH after the addition of 3.00 mL of NaOH.

c) Calculate the pH after the additon of 12.5 mL of NaOH. Notice that this is the half neutralizatiom point: some of the acetic acid molecules are converted to acetate ions producing a buffer whose pH depends on the base/acid ratio (CH3COO-/CH3COOH).

d) Calculate the pH after the addtion of 25 mL of NaOH (equivalence point).

e) Calculate the pH after the addition of 35 mL of NaOH.

f) suggest an indicator other then phenolphthalein that would be suitable for this titration and explain why.

Thank you very much.

1. For the titration of 25.0 mL of 0.1 M HCl (aq) with 0.1 M NaOH (aq), at what volume of NaOH (aq) should the equivalence point be reached and why? If an additional 3.0 mL of 0.1 M NaOH (aq) is then added, what is the expected pH of the final solution?

2. What is the initial pH expected for a 0.1 M solution of acetic acid? For the titration of 25.0 mL of 0.1 M acetic acid with 0.1 M NaOH (aq), at what volume of NaOH (aq) is the equivalence point reached? Is the pH at the equivalence point greater than, less than or the same as in problem #1 above? Explain.

3. What is the initial pH expected for a 0.1 M solution of phosphoric acid (H3PO4)? For 25.0 mL of 0.1M H3PO4 (aq), what volume of 0.1 M NaOH (aq) is required to fully titrate all three protons to their end points?

4. In the titration of a weak acid with a strong base, how is the half equivalence point determined and what is its significance? How are the pKa and Ka of the weak acid determined from the half equivalence point?

5. Carefully read and study the lecture text sections related to this experiment. 5. Prepare and set up your lab notebook. Be sure to leave spaces for recording observations.

CHEM1831 Lecture Notes - Lecture 18: Hexane, Substitution Reaction, Alkene

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