1. If a student forms CaCO3 from the presence of carbon dioxide, but continued with the trial anyway, how
would this affect his/her calculations (too high, too low, or no effect)? Explain your answers.
a) The volume of NaOH required for the back titration.
b) The calculated moles of HCl neutralized by the antacid.
2. A student performed Part II as outlined in the procedure, but when he added the bromophenol blue the
solution turned blue.
a) Why is the solution blue?
b) What can the student do to correct for this error and salvage this trial? How should the data sheet be
modified to reflect this correction?
too high
1a) If a student forms CaCO3 from the presence of carbon dioxide but continues with the trial anyway, it would have no effect on the volume of NaOH required for the back titration. The volume of NaOH required is determined by the amount of unreacted HCl, not by the formation of CaCO3. Therefore, the calculation for the volume of NaOH would not be affected.
b) On the other hand, the formation of CaCO3 would affect the calculated moles of HCl neutralized by the antacid. CaCO3 is a base, and it can react with HCl according to the equation:
CaCO3 + 2HCl -> CaCl2 + CO2 + H2O
The carbon dioxide gas that is produced is lost to the atmosphere. If CaCO3 is formed during the reaction, it would result in an overestimation of the moles of HCl that were actually neutralized by the antacid. Therefore, the calculation for the moles of HCl neutralized would be too high if CaCO3 is formed.
To correct for this error, the student should subtract the moles of CaCO3 formed (determined from stoichiometry) from the total moles of HCl used during the back titration. The moles of HCl neutralized can then be recalculated using the corrected value. The data sheet should be modified to reflect the corrected moles of HCl neutralized.
2a) The solution turned blue because bromophenol blue is a pH indicator that changes color based on the acidity or alkalinity of the solution. In a slightly acidic solution, bromophenol blue appears yellow. As the solution becomes more basic, it transitions to blue. The blue color indicates that the solution is too alkaline.
b) To correct for this error, the student should add a few drops of a strong acid, such as hydrochloric acid (HCl), to the solution containing bromophenol blue. This would neutralize the excess base and bring the solution back to a slightly acidic pH, where bromophenol blue is yellow.
The data sheet should be modified to reflect this correction by noting the addition of the acid to neutralize the excess base and restore the desired pH. Additionally, the volume and concentration of the acid added should be recorded for accurate documentation.