CHEM 113 1nd Edition Lecture 20 Outline of Last Lecture- Review for Exam 2Outline of Current Lecture - Solving Problems Involving Weak-Acid Equilibria- The effect of concentration on the extent of acid dissociation- Polyprotic Acids Current Lecture- Solving Problems Involving Weak-Acid Equilibriao Steps for solving this type of problem 1. Write a balanced equation. 2. Write an expression for Ka 3. Define x as the change in concentration that occurs during the reaction. 4. Construct a reaction table in terms of x 5. Make assumptions that simplify the calculation 6. Substitute values into the Ka expression and solve for x 7. Check the assumptions are justifiedo The notation system Like before, molar concentrations are indicated by [] A bracketed formula with no subscript indicates an equilibrium concentration.o The assumptions [H3O+] from the autoionization of H2O is negligible.- It is so much smaller than the [H3O+] from the dissociation of an acid with a H+ that we can neglect it in these problems. A weak acid has small Ka and its dissociation is negligible- Therefore, it dissociates to such a small extent that we can neglectthe change in its concentration to find its equilibrium concentration.- The effect of concentration on the extent of acid dissociationThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.oo As the initial concentration decreases, the percent dissociation of the acid increases Don’t confuse the concentration of HA dissociated with the percent HA dissociated It is the fraction (or percent) of dissociated HA molecules that increases with dilution Sample Problem 18.9 helps to illustrate this idea.- Polyprotic Acidso A polyprotic acid is an acid with more than one ionizable proton. This means that each dissociation step has a different value for Ka.- In general, these Ka values tend to decrease with each step. Sample problem 18.10 shows you how to use these various Ka values to calculate other
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