Calculations involving?Kc

• In the equilibrium expression each figure within a square bracket represents the concentration in?mol dm-3
• The?units?of?Kc?therefore depend on the form of the equilibrium expression
• Some questions give the?number of moles?of each of the reactants and products at equilibrium together with the volume of the reaction mixture
• The concentrations of the reactants and products can then be calculated from the number of moles and total volume

Equation to calculate concentration from number of moles and volume

Answer

Step 1:?Calculate the concentrations of the reactants and products

Step 2:?Write out the balanced chemical equation with the concentrations of beneath each substance

Step 3:?Write the equilibrium constant for this reaction in terms of concentration

Step 4:?Substitute the equilibrium concentrations into the expression

Step 5:?Deduce the correct units for?Kc

All units cancel out

Therefore,?Kc?=?4.03

Note that the smallest number of significant figures used in the question is 3, so the final answer should also be given to 3 significant figures

• Some questions give the?initial and equilibrium concentrations?of the reactants but not the products
• An initial, change and equilibrium table should be used to determine the equilibrium concentration of the products?using the molar ratio of reactants and products in the stoichiometric equation

Answer

Step 1:?Write out the balanced chemical equation with the concentrations of beneath each substance using an initial, change and equilibrium table

Step 2:?Calculate the concentrations of the reactants and products

Step 3:?Write the equilibrium constant for this reaction in terms of concentration

Step 4:?Substitute the equilibrium concentrations into the expression

= 0.28

Step 5:?Deduce the correct units for?Kc

All units cancel out

Therefore,?Kc=?0.28