For the reaction 2 + 2, the units of Kc are — Chemical Equilibrium Chemistry Question

💡 Solution & Explanation

# Solution: Units of Kc for 2$SO_2$ + $O_2$ ⇌ 2$SO_3$ **Step 1: Write the equilibrium constant expression** $$K_c = \frac{[SO_3]^2}{[SO_2]^2[O_2]}$$ **Step 2: Determine units of concentration terms** Each concentration term has units of mol/L (or M). **Step 3: Calculate the combined units** $$\text{Units of } K_c = \frac{(\text{mol/L})^2}{(\text{mol/L})^2 \times (\text{mol/L})} = \frac{(\text{mol/L})^2}{(\text{mol/L})^3}$$ **Step 4: Simplify** $$K_c = \frac{1}{\text{mol/L}} = \text{L/mol} \text{ or } \text{mol}^{-1}\text{L}$$ **Why A is correct:** Since $\Delta n = n_{\text{products}} - n_{\text{reactants}} = 2 - (2+1) = -1$, the reaction has a decrease in moles of gas. Therefore, $K_c$ must have units of $\text{L/mol}$ (or $\text{L·mol}^{-1}$), which corresponds to option A.