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Thread: Standard Addition Question

  1. #1 Standard Addition Question 
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    I'm trying to figure out this equation which uses Standard Addition. I don't get how they get the final answer considering that there are two unknowns.Standard Addition Q.jpg
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  3. #2  
    Malignant Pimple shlunka's Avatar
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    Dang, I was expecting to see something that I could answer like (10 + 3 = ?)


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  4. #3  
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    A hundred and three!
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  5. #4  
    Malignant Pimple shlunka's Avatar
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    Quote Originally Posted by Neverfly View Post
    A hundred and three!
    Stop using your dictionary against my math.
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  6. #5  
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    Quote Originally Posted by PhDemon View Post
    You have an unknown solution containing Na+ of known volume (95ml), to which you add a known volume (5ml) of standard of known concentration (2.08M). You have measured the signal from the unknown solution and the sample containing the unknown solution and the standard.

    There is only one unknown [X]i (or [Na+] as it appears later). Not sure what your problem is...
    I thought there were two unknowns - [Na+]i and [Na+]f
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  7. #6  
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    Quote Originally Posted by PhDemon View Post
    I think there is a typo in the last equation, both terms should be [Na+]i didn't spot the subscripts before, need my eyesight checked...
    Are you able to show me how they got the final answer.
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  8. #7  
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    Quote Originally Posted by PhDemon View Post
    Not sure how you mean, it's all there in your post. You measure the signal from the solution of unknown concentration [Na+] = 4.27 mV and for the solution after the standard is added (conc = 0.104 + 0.905[Na+]) = 7.98 mV. The ratio of the two signals is equal to the ratio of the two concentrations, this is expressed as the last equation - you just need to solve this for [Na+] using simple algebra.
    I don't get how they got 0.113M
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  9. #8  
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    Quote Originally Posted by PhDemon View Post
    If you are dealing with this technique you really should know enough algebra to solve it but here goes:

    [Na+]/(0.104+0.95[Na+]) = 4.27/7.98

    rearrangeing

    7.98[Na+] = 4.27*(0.104+0.95[Na+])
    7.98[Na+] = 0.44 + 4.06[Na+]

    3.92[Na+] = 0.44
    [Na+] = 0.112 (slightly different as I rounded up the numbers to 2 dp not 3)
    I understand now. Thank you so much. I'm a bit rusty on the algebra plus I would not of spotted the typo in the equation so I would of spent hours trying to figure it out. Thank again!
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    Malignant Pimple shlunka's Avatar
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    Quote Originally Posted by PhDemon View Post
    No worries, glad to help.
    Stop lying.
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  11. #10  
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    Quote Originally Posted by shlunka View Post
    Quote Originally Posted by PhDemon View Post
    No worries, glad to help.
    Stop lying.
    Unless he was using a laptop, he was probably sitting up.
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    Sunuva...

    Fine, then.
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