Ncert Exercises & Solutions

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

For a pure substance (both solids and liquids),

[Pure substance] = \frac{N u m b e r o f m o l e s}{V o l u m e}

= \frac{M a s s / m o l e c u l a r m a s s}{V o l u m e}

= \frac{M a s s}{V o l u m e \times M o l e c u l a r m a s s}

= \frac{D e n s i t y}{M o l e c u l a r m a s s}

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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