Antacids are a class of medications that neutralize acid in the stomach. Determine the moles of HCl, — Titration Chemistry Question

One suggested approach – back titration
* Indication that a back titration with excess HCl will be used to solve the problem
* Students may plan to test titration (observation of endpoint) with the HCl, NaOH, and indicator
* Experimental details on the titration
o Mix tablet, HCl, and indicator and heat the solution to near boiling to release excess CO₂. Remove from heat, allow to cool and test with pH indicator paper to ensure excess acid is present. Titrate with NaOH until the color turns from colorless (or white due to presence of the tablet) to pink.
* Students may attempt to titrate by dissolving the tablet in water and titrating with HCl, but this is an inaccurate method.
o Care should be taken to prevent overheating as the fillers present in the tablet may burn when heating on the hot plate or the water can evaporate.
* Determine the volume of a drop of liquid from a Beral pipette
* Replicate Measurements

Another approach – determine amount of carbon dioxide lost after neutralization
* Indication that measurement of loss of carbon dioxide will be determined by decrease in mass
* Experimental details
o Mass of all items before mixing – making sure to include the glassware for the reaction
o Mass of items after mixing
o Allow time for the reaction to occur – stirring, heating, and waiting until no visible gas remains in the solution and the reaction between the antacid and HCl concluded. Remove from heat, allow to cool, and determine mass after reaction.
o Ensure an excess of HCl was added to the tablet – using pH strips to check the pH.
o Care should be taken to prevent overheating as the fillers present in the tablet may burn when heating on the hot plate or the water can evaporate.
* Replicate Measurements

Back titration
* Recorded mass of antacid tablet used
* Recorded volume of a drop of liquid from a Beral pipette
* Clear data table indicating starting volume of HCl and volume of NaOH used to neutralize the solution
* Observations^- color changes observed in titrations
* Clear indication of how number of drops are converted to volume or how the volume of solutions used was determined

Quantification of carbon dioxide lost after neutralization
* Clear data table including starting and ending masses of antacid, solutions, and glassware and volumes of HCl used
* Observations^- could include the expulsion of gas, the final masses were recorded after all visible gas was removed, pH of the solution after excess HCl was added
* Clear indication of how number of drops are converted to volume or how the volume of solutions used was determined

Back titration
* Calculate the number of moles needed to dissolve the total amount of calcium carbonate in the tablet
* Calculate the total moles of HCl that was used to completely dissolve the antacid tablet and still have excess in solution.
* Calculate the moles of NaOH added to reach the endpoint = moles of excess HCl
* Calculate the moles of HCl that reacted with the calcium carbonate in the antacid tablet
* Average value – from duplicate trials

Quantification of carbon dioxide lost after neutralization
* Mass of carbon dioxide lost
* Mass of calcium carbonate in the antacid tablet
* Calculate the moles of HCl that reacted with the calcium carbonate in the antacid tablet
* Average value – from duplicate trials

* Experimental range: 0.00986 mol – 0.0106 mol HCl per 1.3 g tablet
* Accuracy of value
0.00980 – 0.0106 moles: full credit
0.00882 - 0.00979 or 0.0105 – 0.0116 moles: 5 points
0.00784 - 0.00881 or 0.0117 - 0.0127 moles: 3 points
any other answers (in moles): 0 points
Answers in units other than moles of HCl were converted to moles and then graded using the scale above.