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Determination of antacid strength via titration

Imagine yourself as the Lead Analytical Chemist at Kaplan Industries. Your first big assignment is to investigate the strength of several commercial antacids for the Food and Drug Administration (FDA). They have sent five antacids to be tested with a back-titration that works as follows:
? First, each antacid tablet is mixed with 40 mL of 0.1 M HCl?this acidic solution is the same stuff that is in stomach acid, and one antacid pill isn't anywhere near enough to neutralize all 40 mL of the acid.
? So, to see how much extra help each antacid pill needs to neutralize 40 mL of 0.1 M HCL, you add 0.05 M NaOH drop-by-drop to back-titrate the solution until the pH is neutral.
? What this means is that, the stronger the antacid tablet, the less NaOH it will take to help bring the acid to neutral. (In other words, the stronger antacid tablets counteract more of the original HCl, leaving the solution closer to neutral before the NaOH is added.)
Here are your results:
Maalox Tums Mylanta CVS brand Rennies
Mass of one dose
antacid 20.0 g 21.0 g 18.0 g 18.3 g 17.5 g
mL NaOH used in back-titration 24.1 mL 22.4 mL 20.0 mL 19.9 mL 24.4 mL

1. Which is the strongest antacid, on a single-dose basis? Which is the weakest? Explain and show your calculations.
2. Which are the strongest and weakest, on a by-weight (mass) basis?
3. When people do back titrations, they usually watch the solution for a color change when the solution becomes neutral. What might you have used in the above experiment to get this color change to happen in the solution? At what pH would the solution have been neutral?
4. If you had walked into the lab, only to discover that you only had 0.1 M sulfuric acid available to run your tests, how might this have affected your calculations? Why?
5. In most of the antacids you tested, the active ingredient is Aluminum Hydroxide. Here is an unbalanced reaction that shows how this chemical neutralizes HCl (the main ingredient in stomach acid). Please provide a balanced version of this equation:
Al(OH)3 + HCl --> AlCl3 + H20
6. The FDA requires that all of its reports be super-brief?short enough so that they can be sent via text message to all of its lab sites across the country.. As you probably know, the word limit for text messages is very small, so your goal here is to describe precisely what you did to test the antacids in fewer than 150 words. In this brief report, you should provide the FDA the major findings from your tests and let them know generally how you performed your tests.


Solution Preview

First, technically, this is NOT a back-titration, as you are adding base (antacid) to acid, and then adding MORE base to the acid until you reach the equivalence point. A back-titration would involve going past the equivalence point with base and then "back titrating" with acid until equivalence was reached.

Second, in each case, we are reacting the antacid and base with .040L (the 40mL) of 0.1M HCl, and molarity times volume gives moles as follows: (.040L HCl)*(0.1 mol HCl/L HCl) = .004 mol HCl.
So in all cases, to get to the equivalence point in this reaction of a strong acid with a strong base, we need to have .004 mol of base to fully react in a 1:1 ratio with the acid that's present. Thus, the combination of antacid and base that is added will add up to .004 mol.

1. Since we're adding the antacid first, and then adding the same .05 M NaOH to get to equivalence, whichever antacid neutralizes the most base (ie- is the strongest) will require the LEAST amount of extra base to reach equivalence. So that's the CVS ...

Solution Summary

Data is given from a lab where 5 antacids were added to a set amount of acid and then titrated with base. The strongest and weakest antacids are found from the data, on both a per gram and per dose basis. All work is shown and fully explained. A balanced chemical equation for a normal antacid reaction with stomach acid is also given.