Chemistry

My teacher has us doing a virtual Titration lab on sciencegeek site but never demonstrated a Titration Lab so I really don't undestand it. We have to identify five unknown acids according to how much [H+} of acid is in each solution. We had the choice of adding NAOH in amounts of -05M or .10M or .40M.Then you pick to add base in amounts of 1.00mL or .10mL or .05mL. The three different indicators that can be used are phenolphathalien, methyl orange or thymolphthalien. I picked .40 M of NaOH, added it in amounts of 1.00mL and used phenolphathalien as indicator. I found it changed to color pink when I used 3.80mL of NaOH in the 25.00 of the acid. How do I figure out the [H=] of each of the unknown acids. That is what this lab was about. I just guessed at the amounts of base and the indicator. Is there any more reliable method to selecting the indicator or amount of M or mL of NaOH to be added.

Thank you-I'm really stuck-please help me to understand this

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  1. Certainly I know what a titration is BUT I'm not familiar with a virtual titration experiment.
    The molarity is calculated
    Macid x mLacid = Mbase x mL base provided the reaction is 1:1 such as HCl and NaOH. It can't be done that way if the reaction is not 1:1 such as NaOH and H2SO4 or NaOH and H3PO4. I am assuming you would have started off with the simplest which is 1:1.
    The answer comes out in molarity which is moles/L which would be the H^+ concn.
    I need to know more about the acid/base combination in order to predict the indicator to use. For the amount of titrant to add, if you go too fast (1 mL) you overshoot the equivalence point and if you go too slow (0.05 mL which is about 1 drop) the experiment takes too long.

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  2. I found the sciencegeek page but I didn't find a link to a virtual titration. Sorry I couldn't be of more help.

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  3. The web site for sciencegeek -you need to type in Shockwave/Titration after sceiencegeek


    So when I take the volume of base times the moles of base and divided by volume of acid, that gives me the [H+] of the acid, correct?

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    posted by Shelley
  4. OK. I found the site and did one titration and obtained the right answer so I suppose I did it right. ;-). Here is what you do and I've added some comments at the end.

    Select 0.05 M NaOH
    Add 1.00 mL increments until the indicator changes color, write down the number of mL, then click on redo experiment. On the second try, add 1.00 mL increment until you get to 1 mL LESS THAN the first time, then click on 0.05 mL and add dropwise (0.05 mL) until the color changes, then mLbase x Mbase = mLacid x Macid and calculate M acid. Then you type that answer into the box on the right hand side and it tells you if you are right or not.
    [notes:I didn't play around with the indicators and I didn't change acids (just A) so I don't know if they are 1:1 or not. My best guess is that they are simple titrations and that any indicator will work but that is just a guess. If you have trouble, post and I can go back and try it again. Also, my best guess is that acids A-E are always the same when one checks in; i.e., I get the same acids A-E when I try as you will get when you try but that's just my best guess. If my guess is right, then acid A is 0.06 M for H^+. P. S. the instructions are to type in the number only so .06 gets it without the M]

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  5. Thanks so much-I really appreciate it

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    posted by Shelley
  6. OK. I've run A through E and all of them were correct. You will find that some of them, if you use 0.05 M, will not titrate with 50.0 mL (all the buret will hold) and it will be necessary to redo the titration using 0.1 M or 0.4 M. The obvious choice is to try 0.1 first and if that won't do it switch to 0.4 M NaOH. The procedure is always the same. Make one run clicking fast on 1 mL, restart, add to 1 mL less than what it took the first time, then switch to 0.05 mL (with the same sample), this time adding slow enough that you can stop when the color changes, then do the titration. You must be careful that the box to type in the answer shows the same letter as the acid you are titrating. Mine didn't always show that and I had to mess around with the arrows until I got the right letter in the box, then I typed in my answer. I always had the right answer so I don't know what the hint button does. One other note: I think it times out after 30 minutes and I had to start over, between looking at the site and coming back to Jiskha to give you instructions. Anyway, I hope this takes care of the problem.

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