Exam 3- Worksheet 17 Week 10 Prac. Quiz
Supplemental Instruction
Iowa State University / Leader: / Seth
Course: / Chem 178
Instructor: / Kingston
Date: / Thursday, 3/25/10

Announcements

  1. Exam 3- Wednesday, April 14

Warmup

Titration Strategy- Four Regions

Calculations of pH in titrations of a weak acid with a strong base:

  • Before the addition of a base

HA H+ + A-

Use an ICE table to figure out pH of the weak acid by itself.

  • After adding a base, before the equivalence point

OH- + HA  H2O + A- (reaction goes to completion for number of moles of OH-)

HA H+ + A- (equilibrium after reaction above)

This solution has a weak acid and its conjugate base, so it is a buffer. Use the Henderson-Hasselbalch equation: pH = pKa + log([base]/[acid])

Don’t forget to correct for dilutions with the equation M1V1 = M2V2!

  • At the equivalence point

Here, the number of moles of H+ equals the number of moles of OH-.

OH- + HA  H2O + A- (reaction goes to completion, reacts ALL OH- and HA)

A-+ H2O OH- + HA (equilibrium, start with only A-)

Find the concentration of A- (remembering to account for dilution), and then make an ICE table using the Kb value for the above equilibrium. Solve for pOH and then for pH.

  • After the equivalence point

Here, the base reacts away all the acid and then there is excess OH-:

OH- + HA  H2O + A- (reaction goes to completion, reacts all of HA)

The amount of base in the solution is large compared to the hydrolysis effects of the anion. So, find pOH from the concentration of unreacted OH- and then calculate pH.

  • If your titration is strong acid/weak base, simply switch acid and base components in all of the equations and equilibriums above. (Remember, however, to still plug in Ka, [acid], and [base] exactly where it says in Henderson-Hasselbalch!)

Identifying the reactants in a titration curve:

  • If the curve has an equivalence point at 7, it is a strong acid/strong base titration.
  • If the equivalence point is above 7, the base is stronger than the acid is.
  • If the equivalence point is below 7, the acid is stronger than the base is.
  • Determine what is being titrated by finding the initial pH on the graph. If it is around 1, it is a strong acid; between 2 and 7, it is a weak acid, etc.
  • Determine the titrant by finding the final pH on that graph (see above).

Practice quiz

  1. The solubility-product constant for silver sulfate is 1.5x10-5 at room temperature. What is the molar solubility of Ag2SO4 at room temperature in
  2. Pure water
  3. 0.70 M Na2SO4
  1. You have a beaker with 25 mL of 0.150 M acetic acid and titrate using a buret filled with 0.150 M sodium hydroxide. Given that Ka=6.5x10-5,
  2. Calculate the pH after adding 10 mL NaOH
  3. At the equivalence point