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Homework Help: Using indicators to determine acids, bases, and salts

  1. Sep 15, 2009 #1
    Hey, I just got out of my lab at school and just realized that me and my professor made a mistake. He marked my lab complete, and I also thought it was complete, however, I just realized that there was something missing. The problem is that I can't go back into the lab to try it out again. Its not a big deal I just want to know what would have happened.

    This is the problem:

    In the lab, I added 200mL of water to a 400mL beaker, then I placed it on a magnetic stirrer and added a couple drops of universal indicator, then I added a milk of magnesia tablet. Then I had to note the color changes. Once the tablet completely dissolved, the solution turned purple. Now, the next step was to add a couple drops of HCl to the purple solution. This is the step I forgot, and somehow my prof didn't notice, but I need to know what would happen? Like I said, the lab is already done, however, I don't want this to pop up on the exam or something. So, to recap, my question is, what will happen to the purple solution after I add some HCl?

    My attempt at a solution:

    Now, my assumption is that the solution will become more acidic since the concentration of H+ ions increases:

    H2O + HCl --> H+ + Cl-

    So, since I was using universal indicator, I would assume that as the purple solution becomes more acidic, the color of the solution will change from purple to some lighter color like orange or even red.

    Can anyone help me out? Thanks a lot.
  2. jcsd
  3. Sep 15, 2009 #2


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    Homework Helper
    Education Advisor
    Gold Member

    We really do not know what will happen unless we know the concentration of Milk of Magnesia and the amount used and the concentration of the hydrochloric acid used. "Milk of Magnesia Tablet" is not something for which most people know the concentration which is why we would need to know this. We can guess that "two drops" of HCl means 0.10 ml, but we are not given the concentration of the HCl solution.
  4. Sep 16, 2009 #3


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    Staff: Mentor

    Sounds like antacid titration, which makes me a little doubtful about the story...

  5. Sep 16, 2009 #4
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