This is coming from junior-level physical chemistry.(adsbygoogle = window.adsbygoogle || []).push({});

Lots of thermodynamic properties of reactions involving solutions are tabulated under standard conditions; I'm particularly thinking of standard half-cell voltages, but any property will do.

How do you make, say, a sodium chloride solution under those conditions?

My best understanding is that youcan't. The definition of the standard state of both solute and solvent specifies (1) the pure solvent, which isn't in the sample, and (2) the solute with some hypothetical properties in the infinitely-dilute limit (as determined by the Henry's law constant).

So if my understanding here is correct, what does a standard state property involving solutions even mean? How would you make the solution(s) and measure that property?

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To be a little more concrete, think about it in terms of the equation for chemical potential: [tex] \mu_i = \mu_i^° + RT \ln a_i[/tex]

to get standard conditions the solute must have activity = 1. The formula for activity breaks down to [tex] a_i = \gamma_i c_i/c^° [/tex]

Its common to say "the standard concentration c° = 1 M" but you see in this equation that the nonideality plays a role in the activity coefficient, making the activity NOT unity, and the chemical potential not the standard value.

So what's going on?

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# Can you make a solution under standard conditions?

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