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Homework Help: Why is activity of solvated components equal to 1?

  1. Jan 2, 2018 #1
    1. The problem statement, all variables and given/known data
    Consider the following reaction

    $$\mathrm{Ni^{2+}_{(aq)} + H_2S_{(g)} \to Ni_{(s)} + S_{(s)} + 2H^{+}_{(aq)}}$$

    What are the activities of the terms in the reaction?

    2. Relevant equations

    The activity is defined as ##a = c/c^\circ## in ##\mathrm{ mol \, dm^3}##

    3. The attempt at a solution

    I do not understand why textbook gives ##a_\mathrm{Ni^{2+} (aq)} =1## and ##a_\mathrm{2H^{+} (aq)} =1##: how can I say that those activities are ##1##? Is there a rule to understand activites of solvated compontents (aq)?
  2. jcsd
  3. Jan 2, 2018 #2


    User Avatar

    Staff: Mentor

    It is not always possible, but you can rather easily prepare a solution containing Ni2+ and H+ in such concentrations that their activities equal 1. Makes sense as it is a standard state used as a reference point.

    Something is wrong here, but it is hard to judge without a context. I would expect NiS(s) as a product, not mixture of elements.
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