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When can I use concentrations instead of activities in equilibrium calculations?

To calculate an equilibrium constant, it is said that you should actually use activities $a_i$ instead of concentrations $c_i$. But it is also said that within a certain range, you can use ...
Consider a gaseous state elementary reaction $$\ce{aA(g) + bB(g)} \overset{k_\mathrm{f}}{\underset{k_{\mathrm{b}}}{\ce{<=>}}}\ce{ cC(g) + dD(g)}$$ I know that for this reaction, $$\Delta G = \... 0answers 3k views Why does changing the dilution of an aqueous equilibrium shift the equilibrium? Say we have the equilibrium$$\ce{A_{(aq)} + 2B_{(aq)} <=> C_{(aq)} + D_{(aq)}}$$meaning the formula for the equilibrium constant is$$\ce{K_{c} = \frac{[C][D]}{[A][B]^2}}$$My understanding is ... 0answers 1k views Why are the stoichiometric coefficients used as exponents in the chemical equilibrium constant? From what I can ascertain, the formulation of the chemical equilibrium constant is somewhat arbitrary. I don't understand the motivation for putting the stoichiometric coefficients in the exponents. ... 0answers 59 views Derivation of equilibrium constant [duplicate] In general chemistry, we learn that the equilibrium constant, defined in terms of activities, for the reaction$$\ce{aA + bB -> cD + dD}$$is given by$$K_\mathrm{eq} = \frac{(\ce{C})^c(\ce{D})^...
We know that the rate of a reaction $\ce{aA + bB -> cC + dD}$, the rate of the forward reaction is given by $r_\mathrm f = k_\mathrm f[\ce A]^p[\ce B]^q$ where $a\neq p$ and $b\neq q$ according ...