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Not sure if It is a formula error or a or user error but value in M3 - Q4

  1. #1
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    Not sure if It is a formula error or a or user error but value in M3 - Q4

    Ok so I am still having an issue with my calculator. I have 2 grams of H4EDTA, which has a molar mass of 292.24, this gives me 0.00684369 moles and there is 1 L of water, so this gives me a molarity of 0.00684369 moles per L. No issue there.

    The pKa1 value is 2.07 which gives me a Ka1 value of .008511, pKa2 of 2.75 gives me .001778, pKa3 of 6.24 gives me .000000575439, and pKa4 of 10.34 gives me 0.00000000004571, Again no problem there.

    The problem I am having is in M3-Q4, when take the kA1 value and mutiply it by the moles, I get 0.0085113804 * 0.006844 =0.000058249249809 then when I square it I get an H+ concentration of 0.0076321196.
    In O4 if i just use the same 0.0076321196 then run it through the quadriatic formula i get an num error, if i use a negative of 0.0076321196 I get 0.0832099060

    Both values of 0.0076321196 or 0.0832099060 are WAY too high, as the final value should be at .005642

    When I try use only the 0.0085113804 * 0.006844 =0.000058249249809 without squaring it, and then make it negative, I am getting final value of .004488, which is lower than the required .00564. so what am I doing wrong did i make a mistake in the formula I wrote, do I have something in the wrong spot or is the formula that I am attempting to use incorrectly formed?

    What I am trying to do is a chemistry problem, I am attempting to get an accurate H+ concentration of 2 grams of H4EDTA in 1 L of water so that I can calculate the correct pH.

    Did I miss something???

    Please let me know.
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    Last edited by born2dive00; 12-20-2020 at 10:04 AM.

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    Re: Not sure if It is a formula error or a or user error but value in M3 - Q4

    Interesting challenge!

    Definition of X in Picture 2 give me 0.0176256110429446 in Q17?

    =($N$17+SIGN($N$17))*SQRT(($N$17^2)-(4*$N$16*$N$18))/N16^2

    But no idea how that "Links" to your values in M3 - Q4?

    Ochimus

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    Re: Not sure if It is a formula error or a or user error but value in M3 - Q4

    [.... deleted; waste of time ....]
    Last edited by joeu2004; 12-21-2020 at 05:05 PM.

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    Re: Not sure if It is a formula error or a or user error but value in M3 - Q4

    Disclaimer -- In one of my undergrad chemistry courses, they attempted to teach me these polyprotic acid problems using phosphoric acid, and I failed to get the correct answer every time. The key to these problems is a solid understanding of the chemistry and a good, systematic procedure for solving the problem.

    Reviewing my old chemistry text, I see that I would expect a tetraprotic system to reduce down to solving a quartic (4th order) polynomial and not a quadratic (2nd order) polynomial. Stumbling through your problem, I get a 4th order polynomial solution and I can solve that to give me [H+]=0.005641. I can go into details if needed, but I fear that I took a very different approach than your text/instructor which could lead to more confusion than clarification. At this point, I would suggest that you review the chemistry side of the problem and see if you are supposed to end up with a quadratic equation to solve or a higher order polynomial.
    Quote Originally Posted by shg
    Mathematics is the native language of the natural world. Just trying to become literate.

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