- Mole fractions and partial pressures are a feature of Kp calculations
- Put simply, the mole fraction is the fraction of the total number of moles that each chemical in a reaction is responsible for
- Partial pressure is the part of the total pressure that each chemical in a reaction is responsible for
- The partial pressure of a gas is the pressure it exerts in a mixture of gases if it occupied the container on its own
- Partial pressure is given the symbol p, so for a gas X, it is written as pX
- The total pressure is the sum of the partial pressures (this is known as Daltons' Law)
- To find the partial pressure of a gas, you need two pieces of information
- The total pressure in the container
- The mole fraction
- The mathematical relationships are as follows
Partial pressure and mole fraction expressions
- When dealing with equilibrium calculations there are certain calculations that you will be expected to be able to perform
- Calculating concentrations
- Concentration (mol dm-3) =
- Calculating equilibrium quantities
- Calculating mole fractions and partial pressures
- Calculating concentrations
Ethanoic acid and ethanol react according to the following equation:
CH3COOH (I) + C2H5OH (I) ⇌ CH3COOC2H5 (I) + H2O (I)
At equilibrium, 500 cm3 of the reaction mixture contained 0.235 mol of ethanoic acid and 0.035 mol of ethanol together with 0.182 mol of ethyl ethanoate and 0.182 mol of water.
Calculate the concentration of each chemical at equilibrium.
- [CH3COOH] 0.470 mol dm-3
- [C2H5OH] 0.070 mol dm-3
- [CH3COOC2H5] 0.364 mol dm-3
- [H2O] 0.364 mol dm-3
Calculating equilibrium quantities
- 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
Ethyl ethanoate is hydrolysed by water:
CH3COOC2H5(I) + H2O(I) ⇌ CH3COOH(I) + C2H5OH(I)
0.1000 mol of ethyl ethanoate are added to 0.1000 mol of water. A little acid catalyst is added and the mixture made up to 1dm3. At equilibrium 0.0654 mol of water are present.
Use this data to calculate the equilibrium concentrations of each chemical.
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
- [CH3COOC2H5] 0.0654 mol dm-3
- [H2O] 0.0654 mol dm-3
- [CH3COOH] 0.0346 mol dm-3
- [C2H5OH] 0.0346 mol dm-3
Calculating mole fractions and partial pressures
- These are two of the fundamental calculations associated with Kp calculations
Working out mole fractions
A sample of 0.25 mole of nitrogen and 0.75 mole of hydrogen were reacted together to form ammonia. The equilibrium amount of nitrogen was 0.16 mol.
N2 (g) + 3H2 (g) ⇌ 2NH3 (g)
Calculate the mole fractions of nitrogen, hydrogen and ammonia.
Write out the equation and record the initial, the change and the equilibrium amounts:
You can check you have the mole fractions correct by adding them up and making sure they come to 1: 0.195 + 0.585 + 0.220 = 1
The total pressure for the reaction, described above, of nitrogen and hydrogen to form ammonia was 150 kPa.
Calculate the partial pressure of each gas.
- Partial pressure of N2 (g) = 0.195 x 150 = 29.25 kPa
- Partial pressure of H2 (g) = 0.585 x 150 = 87.75 kPa
- Partial pressure of NH3 (g) = 0.220 x 150 = 33.0 kPa
- You can check that your partial pressures are correct by adding them up:
- 29.25 + 87.75 + 33.0 = 150
- They should add up to the total pressure - if they don't, then there is at least one calculation wrong somewhere!