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ISU CHE 141 - Aqueous Equilibrium
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CHE 141 1st Edition Lecture 20 Current LectureAcid-Base Titrations- Titrant: the standard solution added to the sample in a titration - Analyte: the substance of interest in an analytical procedure- Equivalence point: the point in a titration where the number of moles of titrant added is stoichiometrically equal to the number of moles of the substance being analyzed- Acid-base titration: a solution of known concentration of base is slowly added to the acid- The pH is monitored with a pH meter or an indicator- Indicator substance with color dependent on the pH- As the acid and base combine they neutralize each other- Equivalence point where the number of moles of base is stochiometrically equal to the number of moles of acid- The graph of pH as a function of titrant added is called pH titration curve- The exact shape of the pH titration curve depends on several factors, including thestrength of the acid or base being titrated- From the shape of the pH titration curve we can determine the equivalence point, select suitable indicators and determine Ka for a weak acid- We will examine the curves for the followingThese 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.- Strong acid-strong base- Weak acid-strong base- Polyprotic acid-strong baseTitration of a Strong Acid with a Strong Base- 1. Initial pH: Only strong acid present before titration so pH=-log[acid]- 2. Between initial pH and equivalence point H+ consumed as OH- added, formingH2O (pH<7)- 3. Equivalence point: H+ completely neutralized by OH- (pH=7)- 4. After the equivalence point: No H+ left to react with excess OH- (pH>7)- Rules for titration of a strong acid with a strong base- The initial pH is the pH of the strong acid to be titrated- Before the equivalence point, calculate [H3O+] by subtracting the number of moles of added OH- from the initial number of moles of H3O+ and dividing by the total volume- At equivalence point, pH=7- Beyond equivalence point, calculate [OH-] by subtracting the initial volume of moles of H3O+ from the number of moles of added OH- and dividing by the total volumeTitration of a Weak Acid with a Strong Base - 1. Initial pH: Only weak acid present, use Ka to calculate pH- 2. Between initial pH and equivalence point: added OH- converts HA into A-, forming buffer solution- 3. Equivalence point: HA completely neutralized by added base, pH base on reaction of conjugate base with water- 4. After equivalence point: No HA left to react with excess OH-- Rule: the volume at the equivalence point in an acid-base titration does not depend on whether the acid being titrated is a strong acid or weak acid, it dependsonly on the amount (in moles) of acid present in solution before the titration begins, the concentration of the added base- The pH of the equivalence point will depend on whether the acid/base being titrated is strong or weak- Note that the pH at the equivalence point is not neutral but basic- The titration of a weak acid by a strong base always has a basic equivalence point because at the equivalence point all of the acid has been converted into its conjugate base, resulting in a weakly basic solution- Similarly, the equivalence point for the titration of a weak base by a strong acid is acidic as all of the base has been converted into its conjugate acid, resulting in a weakly acidic solution- Rules for titration of a weak acid with a strong baseo Initial pH is that of the weak acid solution to be titratedo Before equivalence point, solution is a buffer. Use reaction stoichiometry to calculate the amounts of each buffer component, then the Henderson Hasselbalch equation to calculate pHo Halfway to the equivalence point, the buffer components are equal and pH=pKao At equivalence point, calculate pH by working an equilibrium problem forthe ionization of water by the conjugate baseo Beyond equivalence point, calculate [OH-] by subtracting initial number of moles of weak acid from the number of moles of added OH- divided bytotal volume- 1. Initial pH: only weak base present, use Kb to calculate pH- 2. Between initial pH and equivalence point, added H+ converts B into HB+ forming buffer solution- 3. Equivalence point: B completely neutralized by added acid, pH based on reaction of conjugate acid with water- 4. After equivalence point: No B left to react with excess H+Titration of a Polyprotic Acid with a Strong base - When a diprotic acid is titrtated with a strong base, if Ka1 and Ka2 are sufficiently different we will see two equivalence points , one for each protonpH Indicators- A pH indicator is a weak organic acid that is a different color from its conjugate base- The color of a solution containing an indicator depends on the relative concentrations of HIn and IN-- If [In-]/[HIn]=1, solution is intermediate in color- If [In-]/[HIn] greater than 10, solution is color of In-- If [In-]/[HIn] less than 0.1, solution is color of


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ISU CHE 141 - Aqueous Equilibrium

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