For the reaction A(g) plus B(g) forming 2C(g), a 2 dm cubed vessel at equilibrium holds 0.40 mol of A, 0.20 mol of B and 0.80 mol of C. What is the equilibrium constant?

1. The equilibrium constant is built from concentrations, so the moles must be divided by the volume first. 2. The concentration of A is 0.40 moles divided by 2 cubic decimetres, which is 0.20 moles per cubic decimetre. 3. The concentration of B is 0.20 divided by 2, which is 0.10 moles per cubic decimetre. 4. The concentration of C is 0.80 divided by 2, which is 0.40 moles per cubic decimetre. 5. Products go on top and reactants underneath, and each concentration carries the power of its coefficient. 6. C has a coefficient of two, so the top of the expression is 0.40 squared, which is 0.16. 7. The bottom is 0.20 multiplied by 0.10, which is 0.020, so the constant is 0.16 divided by 0.020, giving 8. 8. The value of 16 is the trap for doubling the concentration of C instead of squaring it, the value of 80 forgets to convert moles into concentrations, and the value of 4 leaves the square off altogether. _Source: Siyavula Physical Sciences Grade 12 (Everything Science, CC BY 4.0), Chapter 8: Chemical equilibrium_