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I learnt that we use activity to define the equilibrium constant . But while calculating the equilibrium constant we plug in their concentration instead of activity .

For a general reversible reaction $$\ce{aA + bB <=> cC + dD}$$ $$K=\frac{[C]^c [D]^d}{[A]^a [B]^b}$$ We put their concentration instead of activity. Why is that done ?

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While calculating concentrations is trivial (divide the amount of substance by the volume), activity is a complete different story. Activity is usually calculated by multiplying the concentration by a dimensionless number called activity coefficient($\gamma$): $$a = c \times \gamma$$

The problem here is that $\gamma$ is not only different for every substance, it also varies with temperature, concentration of the substance itself, the diluent used, and so on. If the substance is a salt that dissociates then the calculation gets even more complex.

Since having the values of the activities would be quite a challenge, equilibrium constants are often written using concentrations, which are more readily available. It's not the most correct way of represent it, but it makes your life easier.

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