Experiment 1- Qualitative Analysis of Cations
TA: Yu-Chun Lin
September 25, 2017
Results & Discussion
This experiment was composed of three parts. In the first part, qualitative methods were used to identify the unknown cation in an aqueous solution. The cation was to be either lead or silver. If silver was present, extra steps were needed to be taken in order to confirm its presence in the solution. In the second part of the experiment, qualitative methods were used again to determine the identity of the other cation in the original aqueous solution of the first part. The cation was to be either barium or calcium. In this part, a false positive …show more content…
If this experiment were to be repeated, another cation could be added for identification or a flame test could have been done with the solutions in Part A and Part B in order to determine the cation identity. In Part A of this experiment, after carefully following each procedure, lead was concluded to be the cation present in the #2 unknown aqueous solution. This was concluded because of the formation of a yellow powdery precipitate inside the solution during the sixth step. This was as expected. In Part B of this experiment a few mishaps occurred: the pH paper was accidentally placed in the test tube in step ten, and more than the recommended water input was added to the test tube in step eleven. More water was added in step eleven because the solution was too cloudy. The results of Part B were interesting because it was expected that there could only be calcium or barium present in the solution, but both were confirmed present. A precipitate formed in both of the tubes that were needed to be used to confirm the presence of barium and calcium.
The confirmation of barium, however, could have been the result of a false positive. In Part C, …show more content…
It is necessary to add HCl to the unknown solution at the beginning because it is a strong acid so it easily dissociates in the solution and the chlorine anion can react with the cation present in the solution. In step 5, water was added to the solution and then the test tube was heated so that the bonds holding the compounds in the precipitate together could effectively break. PbCl₂ has a higher water solubility, so if a yellow precipitate formed, then that would have confirmed that lead was present. If not, then extra steps were needed to be taken to confirm the presence of silver.
In step 12, K₂CrO₄ was added because if barium was present, it would react with the compound to form BaCrO₄. This was necessary in order to determine whether or not our solution had produced a false positive.
A “false positive” occurs when a test incorrectly indicates that a particular condition or substance is present. In step 12, the formation of a precipitate could have been due to the presence of barium or it could have been a false positive, causing us to think that barium was present.
A centrifuge is a machine that uses a strong centripetal force to cause denser materials to form