a. Weak Base Titration

Overview

In the first part of this experiment data was streamed over the web for the titration of a weak acid by a strong base that was performed by your teaching assistant.  In this part you are going to use the virtual lab to titrate a weak base with a strong acid.  Students should operate in pairs with one student recording the volume titrant and pH of the solution in a Google Sheet and the second running the virtual lab.  Each group is assigned an unknown by their instructor and the can obtain the unknown by going to the bottom of the stockroom  where they can obtain 100 mL of 1M unknown (figure \(\PageIndex{1}\).  Even though we could safely titrate any concentration in the virtual lab we want you to dilute all stock solutions to 0.1 M.

Note, you are titrating a weak base and are seeking K_b and the Henderson Hasselbach eq. is

\[pOH=K_b + log \frac{salt}{base}\]

and your titration curve is reading pH and not pOH.

Assignment

Before starting you titration in the virtual lab you need to design your experiment and figure what volumes of analyte and titrant you should use.  The virtual lab gives you the concentration of each reagent in the stock room, which is what we are calling the Stock solutions.  These are 1M or greater and although you could titrate 1 M solutions in a virtual lab we want you to dilute them to 0.1 M like you would in the real lab.  So Question 1 deals with diluting the stock analyte and titrant solutions to safe concentrations to perform the titrations.  The second question is central to the design of the titration as you need a lot of data around the equivalence point. 

In this lab you need to titrate 25 mL of the analyte and make pH measurements every 1 mL except when you are near the equivalence point, where you need to take make a measurement every 0.1 mL for the range from one mL below to one mL above the equivalence point.

Question 1: Dilution of Stock Solutions

You have stock 1 M HCl and 1 M unknown solutions and need to make 100 mL of each solution to use in the titration.  Indicate the quantity of reagent (HCl or unknown) that would need to be added to a 100 mL volumetric flask so the final solution would be 0.1 M, and then add that to a 250 mL Erlenmeyer flask. Record this value in the Google Sheet  

Question 2: Predict Equivalence Point

Since you know the concentration of the acid and base you can predict the volume of base required to neutralize 25 mL of the acid.  Insert this value into the Google Sheet, and you can change it later if you discover it is wrong, but you need to know this value in order to make small measurements around the equivalence point.  You will not be required to do derivative plots in these labs as you "know" the equivalence point.

Figure \(\PageIndex{1}\): Copy and Paste Caption here. (Copyright; author via source)

 

3. Calculate K_B from the titration curve (using the value of half equivalence).  Remember that the Henderson Hasselbach equation is written in terms of hydroxide for a weak base.

 

Virtual Lab

This virtual lab should load the "Titration of Weak Base" lab, if it says "Default" you need to refresh the page.

Figure \(\PageIndex{2}\): Be sure the correct lab loads