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I want to know why concentration of solids and liquids are taken unity in expression of equilibrium constant( heterogenous equilibrium). It is usually mentioned that the density of solids and liquids( ideally) does not change and hence concentration does not change and so rate does not depend on solids and liquids. I accept that it is true. But will not it change the actual value of $ K_c$? We use $K_c$ everywhere like calculating extent of reaction, in thermodynamics- calculation of standard gibbs free energy and so on...

How the assumption of unit concentration in expression is justified?

I want to give you an example: Suppose in expression of $K_c$ the concentration of water(liquid) is involved then the concentration wil be $(1000 g/L)/(18g/mol)$ ( density/ gram molar mass) Now if we take the concentration of water(liquid) unity then definitely value of $K_c$ will change and this change will be reflected. for example: when we want to predict extent of reaction $OR$ put value of $K_c$ in different equations relating it like $G^○=-RTlnK_c$

Kartoos
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  • Check this: http://dlrgenchem.com/LECTURES/omitsolids.htm – Molx Mar 29 '15 at 04:37
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    This doesn't look like a duplicate. The linked question talks about constant concentration while this is about taking unity as the concentration of solids and liquids in calculations. – Del Pate Mar 29 '15 at 17:11

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