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Homework Help: Gravimetric Analysis to calculate wgt%?

  1. Jan 28, 2012 #1
    A solid 0.5330-g sample which contains only AgNO3(s) and Hg2(NO3)2(s) is dissolved in aqueous medium and the metal cations are precipitated as a mixture of the respective insoluble cobalticyanide salts.
    3Ag+(aq) + Co(CN)63−(aq) → Ag3[Co(CN)6](s)
    3Hg22+(aq) + 2Co(CN)63−(aq) → (Hg2)3[Co(CN)6]2(s)
    The washed and dried precipitate has mass 0.5571 g.
    What was the wt% of AgNO3 in the original sample?

    I tried to solve this using (weight precipitate)*(gravimetric factor)(100) / weight of sample but i got the wrong answer!? can someone please shed some light on this for me !?
  2. jcsd
  3. Jan 29, 2012 #2


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    Staff: Mentor

    Which gravimetric factor? Your precipitate is a mixture of TWO solids.

    The idea behind this type of the question is to write two independent equations - for example assume nAg (number of moles of Ag) and nHg2 are your unknowns. Express masses of the initial and final mixtures using these numbers of moles (and respective molar masses) and you have two equations in two unknowns - just solve.

    But I have a feeling it won't work as expected in this case, as the system of equations is ill-conditioned (or perhaps they are even just dependent).
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