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Explain Why Pure Liquids and Solids Can Be Ignored While Writing the Equilibrium Constant Expression? - Chemistry

Explain why pure liquids and solids can be ignored while writing the equilibrium constant expression?

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Solution 1

For a pure substance (both solids and liquids),

`["Pure substance"] = "Number of moles"/"Volume"`

`= "Mass/molecular mass"/"Volume"`

`= "Mass"/("Volume x Molecular mass")`

`= "Density"/"Molecular mass"`

Now, the molecular mass and density (at a particular temperature) of a pure substance is always fixed and is accounted for in the equilibrium constant. Therefore, the values of pure substances are not mentioned in the equilibrium constant expression.

Solution 2

This is because molar concentration of a pine solid or liquid is independent of the amount present.

`"Molar concentration" = "No of moles"/"volume" xx "Mass"/"volume" xx "Density"`

Since density of pure liquid or solid is fixed and molar mass is also fixed. Therefore molar concentration are constant.

Concept: Equilibrium in Physical Processes - Solid-liquid Equilibrium
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NCERT Class 11 Chemistry Textbook
Chapter 7 Equilibrium
Q 7 | Page 225
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