Why is activity of solvated components equal to 1?

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1. Jan 2, 2018

crick

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. Jan 2, 2018

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.