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Homework Help: Equilibrium chem lab (FeSCN2+)

  1. Mar 1, 2010 #1
    1. The problem statement, all variables and given/known data

    Chem lab on equilibrium. We added the same amount of Fe(NO3)2 (5ml) to varying amounts of KSCN and used a given Beer's law and absorption data to calculate concentration of FeSCN2+ and then calculate the Eq. constant (K).

    I calculated the Eq. constant, but I'm stuck on two follow up questions:

    Consider the equation Fe+3(aq) + SCN(-)(aq) <---> FeSCN 2+

    Which ion (Fe3+) or SCN (-) is never in excess? Why?
    What experimental conditions are needed to ensure "all" SCN(-) is present as FeSCN2+

    2. Relevant equations

    The intro we were given in our lab manual described the instability of Fe2+ in water (how it forms Fe3+). Also described the colors of Fe and FeSCN2+. Also defined complex ions and oxidation states.

    3. The attempt at a solution

    I don't understand how I would know which ion is limiting. We used varying amounts of KSCN and kept the amount of FeNO3 consistent. The stoichiometry would indicate it's a 1:1 ratio. According to the calculations I did for one trial, I end up with fewer moles of SCN(-) at equilibrium, so does that mean it's limiting?

    Any guidance on how to figure this out would be appreciated.
  2. jcsd
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