Chemistry Question

Overview

• On a piece of paper (or on your tablet), complete the following:

Part 1:

1. 1. Randomly choose a number between 2-5 and another number between 7-12. These two numbers represent the pKas for a generic (imaginary) polyprotic acid with TWO dissociable protons (H₂A) where pKa1 is between 2-5 and pKa2 is be between 7 and 12.
   2. Write the equations/equilibria for the dissociation of this generic acid (H₂A). Write your pKas above the arrows.
   3. Graph/sketch the titration curve for this generic acid (H₂A). Your X-axis should be OH- equivalents and your Y-axis should be pH. Be sure the graph MATCHES that of your 2 chosen pKas for this generic acid.
      • Label the following on your titration curve graph:
        • X- & Y-axes (numbers & labels)
        • Eqivalence points (location on curve)
        • pKas (location on curve & pH at that location)
        • Location on curves of the following:
          • 100% H₂A
          • 100% HA^-
          • 100% A^2-
          • [H₂A]=[HA^-]
          • [HA^-] = [A^2-]
        • Where on the curve is the most acidic vs. most basic.

Part 2:

1. 1. Choose a pH between 2-12 that is NOT equal to any of the pKas of your generic acid (that you had chosen for Part 1).
   2. Draw an "X" on your titration curve (from Part 1) showing the location of your chosen pH.
   3. What is the (approximate) fractional concentration of the two most common species at this pH? Assume concentrations of the other species are negligible.
      • Write out ALL your equations used to solve this problem (e.g., Henderson-Hasselback).
      • Show all your work and calculations clearly (so a reader can follow your logic and calculations).
      • Highlight or draw a box around your final answer.

• See Table 2.4 and/or Fig. 2.13 titration curve for phosphoric acid as an example of a polyprotic acid with THREE dissociable protons. Question 9 in the study problems is similar (using phosphoric acid), so you can use that as a reference.
• respond to AT LEAST TWO of your classmates by (1) checking their calculations (2) highlighting areas where you agree and can follow their work, and/or (3) trying to resolve where you disagree or where you were unclear on their calculations.
• Be courteous and constructive with your comments. You can politely correct misconceptions or errors, or point out areas for clarification.