The Interdisciplinary Laboratory

 Pre-Lab Assignment for Experiment #2:
Carbonate Content of Biological Hard Tissue

The following titration data were obtained for the analysis of the calcium carbonate content of a sample of oyster shell.  Determine the weight percent of calcium carbonate in each sample, and compute the average value and average deviation.
 
 

Using the Sartorius balance, my lab partner weighed out three samples of oyster shells.
The weights of each sample were:
 
 
Sample #1 0.1705 ± 0.0001 g
Sample #2 0.1734 ± 0.0001 g
Sample #3 0.1513 ± 0.0001 g

I transferred the samples to a 250-mL Erlenmeyer flask, rinsing the weigh boats with a few mL of deionized water to obtain a complete transfer.

 We then added 25.0 ± 0.1 mL of a 0.204 M HCl solution to each flask using a 25-mL TD volumetric pipet.  The samples did not dissolve immediately, but we did notice some bubbling.  Must be the evolution of CO2 (g).

 Next we added a stir bar to each flask and placed on the heater/stir plate, heating and stirring.

 After 5 minutes still solid left; turned up heat.

 After 10 minutes solid disappearing, no boiling yet.

 After 15 minutes, only filmy material left.  Removed the samples from the hot plate to cool on their own.

 My lab partner rinsed and filled the buret with 0.105 M NaOH solution. (No uncertainty noted.)  Now to titrate the samples!  I'll titrate each sample for consistency.  I first will add two drops of phenolphthalein to each flask before titrating.
 
 


Buret reading at start / mL Final buret reading / mL Observations
Sample #1 3.55 ± 0.02 22.30 ± 0.02 Faint pink color - easy to detect
Sample #2 22.30 ± 0.02 45.25 ± 0.02 Matched endpoint color to Sample #1
Sample #3 0.90 ± 0.02 20.10 ± 0.02 Refilled buret; waited for liquid to drain from the walls before reading starting volume.
Matched the pink color again!

Submit via e-mail the numerical solutions to this pre-lab assignment (the weight percent of calcium carbonate in each sample and the computed average value and average deviation) to Professor Van Hecke (Gerald_VanHecke@hmc.edu) by 7 p.m. on the Monday evening prior to your first laboratory day (i.e., Monday, September 8 or Monday, October 6).