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