Concepts in Acid-Base Titrations

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. (The concentration of an unknown titrant can also be determined by titration with an analyte of known concentration and volume.) Titration curves (graphs of volume vs. pH) have characteristic shapes. The graph can be used to determine the strength or weakness of an acid or base. 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. There is a steep incline or decline at this point of the titration curve. It is also common to use an indicator that changes color at or near the equivalence point. In this assignment, you will observe this titration curve by titrating the strong acid HCl with the strong base NaOH. 

1. Start Virtual ChemLab, select Acid-Base Chemistry, and then select Concepts in Acid-Base Titrations from the list of assignments. The lab will open in the Titration laboratory.

2. Click the Lab Book to open it. The buret will be filled with NaOH. The horizontal position of the orange handle is off for the stopcock. Click the Save button in the Buret Zoom View window. 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 35 mL, double-click the stopcock to stop the titration. Click Stop in the Buret Zoom View. A blue data link will be created in the lab book, click on it to view the data.

3. The beaker contains 0.3000 M HCl and the buret contains 0.3000 M NaOH.

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

NaOH + HCl = NaCl + H2O

The following questions can be answered by examining the Plot and Data Viewer windows.

5. What was the pH and color of the solution at the beginning of the titration?  0.91 & yellowish

6. What was the pH and color of the solution at the end of the titration? 12.44 & blueish

7. Examine the graph of the pH vs. volume (blue line). 

Sketch the shape of the titration graph of pH vs. volume.

8. What happens to the pH around 25 mL?  it rises  from 3.12 pH to 10.96 pH .

9. What would cause the change observed in question #4?