Chem 1403                                                                  _____________________

Virtual Prelab Exp. 8a                                                                           (name)


Third Lab: Potentiometric Titration of Unknown Acid




This Lab can be accessed at following Link:


If you forgot your password, type in any password, and then follow the register again link.  Once you get to the site, open up the Third Virtual Lab (Potentiometric Titration of Unknown Acid) and determine the concentration of your unknown acid.  If you forgot your unknown number it is posted on the classroom bulletin board.


This is a prelab activity designed to help you run the lab. You will make two titration curves from virtual data.  In the real lab you will first do a cowboy run to get a feel for the titration curve.  Then based on the cowboy run you will do a very accurate lab which you will use to calculate the equilibrium plot of your unknown.  This prelab will follow that protocol.


Part 1: Cowboy run:  Place 20 mL of your unknown 1.0 acid in a beaker and add 2 drops of phenolphthalein indicator.   Add  25 mL of 1.0 M NaOH  in 1.0 mL increments and record pH after every 1.0 mL increment  Make a titration curve of the above data. 


Part 2:  Since you are titrating  20 mL of 1.0 M unknown acid with 1.0M NaOH you know the equivalence point will occur at 20 mL of NaOH.  So, this time you will not take a lot of data in the buffer (flat) region of the curve (but need data around half equivalence).   So record data at 0.0, 5.0, 9.0,10.0, 11.0 15.0 19.0, and 19.5 mL.  Then make recordings every 0.1mL from 19.5 to 20.5, then measure at 21 and 23 and 25 mL.


Print both graphs.  Calculate the Ka based on the pH at half equivalence from the data in graph 2.


Hand in this sheet and your essay typed to the back:


a. Unknown number________


b. Ka of your unknown acid______


c.  Very brief description of how you determined Ka.  Include any calculations.