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Buffers

Buffers. Buffer Capacity. Buffer capacity represent the amount of protons or hydroxide ions the buffer can absorb without significantly changing the pH.

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Buffers

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  1. Buffers

  2. Buffer Capacity. Buffer capacity represent the amount of protons or hydroxide ions the buffer can absorb without significantly changing the pH. The pH of a buffered solution is determined by the ratio of [A-]/[HA]. The capacity of a buffered solution is determined by the magnitudes of [HA] and [A-].

  3. Adding Strong Acid to a Buffered Solution II. Calculate the change in pH that occurs when 0.0100 mol gaseous HCl isadded to 1.0-L of each of the following substances: Solution A: 5.00 M HCH3COO and 5.00 M NaCH3COO Solution B: 0.050 M HCH3COO and 0.050 M NaCH3COO Solution C: 0.020 M HCH3COO and 0.020 M NaCH3COO For acetic acid, Ka = 1.8 x 10-5

  4. Preparing Buffers The most effective buffers have a [A-]/[HA] ratio of one pH = pKa + log ([A-]/[HA] ) The pKa of the weak acid used should be as close as possible to the desired pH of the buffered solution.

  5. Preparing a Buffer A chemist needs a solution buffered at pH 4.30 and can choose from the following acids and their salts: chloroacetic acid (Ka = 1.35 x 10-3) propanoic acid (Ka = 1.3 x 10-5) benzoic acid (Ka = 6.4 x 10-5) hypochlorous acid (Ka = 3.5 x 10-8) Calculate the ratio [HA]/[A-] required to yield the pH 4.30. Which system works best?

  6. Preparing a Buffer II How would you prepare a benzoic acid/benzoate buffer with pH 4.25,starting with 5.0-L of 0.050 M sodium benzoate (NaC6H5COO) solution and adding the acidic component? Ka of benzoic acid (C6H5COOH) = 6.3 x 10-5.

  7. Titration and pH Curves.

  8. Titration and pH Curves. A titration curve is a plot of pH vs. volume of added titrant.

  9. Titrations Because titrations involve small concentrations, and mL are of often used in titrations, and millimoles, or mmol. Molarity = mmol/mL The equivalence point is when [H3O+]=[OH-]. All volumes in a titration are considered to be additive. Always label the equivalence point and the point where pH = pKa.

  10. Strong Acid-Strong Base Titration Curves. Before the addition. pH is calculated directly from the initial concentration. Additions before the equivalence point. Construct a “stoichiometry” reaction table. Determine MOLES of acid in excess (not neutralized). Divide MOLES by the TOTAL VOLUME to obtain [H3O+]. Calculate the pH.

  11. Strong Acid-Strong Base Titration Curves. Additions at the equivalence point. The pH ALWAYS is equal to 7.00 when [H3O+] = [OH-]. Additions beyond the equivalence point. Construct a “stoichiometry” reaction table.Determine MOLES of base in excess (not neutralized).  Divide MOLES by the TOTAL VOLUME to obtain [OH-].  Calculate the pOH, then the pH.

  12. Problem 50.0 mL of 0.200 M HNO3 are titrated with 0.100 M NaOH. Calculate the pH after the additions of 0.0, 10, 20, 50, 100,150, and 200 mL samples of NaOH. Then, construct a titration curve and labelit properly.

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