# 1.7.4 Equilibrium Constant Calculations

### Equilibrium Constant: Calculations

#### 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

• 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 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

#### • Step 1: Write out the balanced chemical equation with the concentrations of beneath each substance using an initial, change and equilibrium table #### Calculations involving Kp

• In the equilibrium expression the p represent the partial pressure of the reactants and products in Pa
• The units of Kp therefore depend on the form of the equilibrium expression
• Some questions only give the number of moles of gases present and the total pressure
• The number of moles of each gas should be used to first calculate the mole fractions
• The mole fractions are then used to calculate the partial pressures
• The values of the partial pressures are then substituted in the equilibrium expression
• Step 1: Calculate the total number of moles

Total number of moles = 1.71 x 10-3 + 2.91 x 10-3 + 1.65 x 10-2

= 2.112 x 10-2

• Step 2: Calculate the mole fraction of each gas • Step 3: Calculate the partial pressure of each gas

H2 = 0.0810 x 100 = 8.10 kPa

I2 = 0.1378 x 100 = 13.78 kPa

HI = 0.7813 x 100 = 78.13 kPa

• Step 4: Write the equilibrium constant in terms of partial pressure • Step 5: Substitute the values into the equilibrium expression = 54.7

• Step 6: Deduce the correct units for Kp All units cancel out

Therefore, Kp = 54.7

• Other questions related to equilibrium expressions may involve calculating quantities present at equilibrium given appropriate data

There are equal volumes of reactants A and B in a 1:1 molar ratio.
This means their partial pressures will be the same.
B therefore also has an equilibrium partial pressure of 0.5

Total pressure = sum of equilibrium (Σ) partial pressures

Therefore, the sum of all the partial pressures must equal to 3 atm

0.5 + 0.5 + pc = 3 atm

pc = 2 atm

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