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6-11: Weak Acid-Strong Base Titrations

 

In this video we'll show you how to operate the buret:

Titrations provide a method of quantitatively measuring the concentration of an unknown solution. In an acid-base titration, this is done by delivering a titrant of known concentration into an analyte of known volume. Titration curves (graphs of volume vs. pH) have characteristic shapes. The equivalence point of the titration, or the point where the analyte has been completely consumed by the titrant, is identified by the point where the pH changes rapidly over a small volume of titrant delivered. In this assignment, you will observe this titration curve by titrating the weak acid CH3COOH (acetic acid) with the strong base NaOH. You will also predict the pH at the equivalence point, validate your prediction experimentally, and then calculate the equilibrium constant for the neutralization reaction.

 

  1. To start this activity, click this link for Weak Acid-Strong Base Titrations. The lab will load in a new tab. Click back to this tab to read further instructions and complete the questions below. You can follow along with the instructions below in the Procedures tab in the lab.
  1. The buret will be filled with 0.2006 M NaOH and 25.00 mL of 0.1894 M CH3COOH will be in the beaker with phenolphthalein as the indicator.

Write a complete balanced equation for the neutralization reaction between CH3COOH and NaOH.


3. Now write a balanced net ionic equation for this chemical reaction.


  1. Indicate the species present at the equivalence point, and predict whether the pH at the equivalence point will be pH > 7, pH < 7, or pH ≈ 7. Explain why you made this prediction.

  1. Click the Save Data button in the Buret section of the Live Data tab. The horizontal position of the orange handle is off for the stopcock. Open the stopcock by pulling down on the orange handle. The vertical position delivers solution the fastest with three intermediate rates in between. Turn the stopcock to one of the fastest positions. Observe the titration curve. When the volume reaches 40 mL, double-click the stopcock to stop the titration. Click Stop Saving to stop recording the data to the Lab Book.

      If you need to repeat the titration, click on the link above again or click the link in the Presets tab for Lab Preset: Weak Acid-Strong Base.

  1. Examine the graph of pH vs. volume  and attach the titration curve below. Take a screenshot and mark on it the equivalence point as halfway between the top and bottom “shoulders” of the curve.

7. Based on your graph, what is the pH at the equivalence point?

How does it compare with your predicted pH?


  1. Calculate the value of the equilibrium constant for the balanced net ionic equation for the weak acid-strong base titration. Show your work.