How do you calculate the partial pressure of a gas?

Mole fraction of the gas × total pressure.

How do you calculate a mole fraction?

Moles of the gas ÷ total moles of gas in the mixture.

Write Kp for N2(g) + 3H2(g) \rightleftharpoons 2NH3(g).

Kp = \frac{p{NH3}^2}{p{N2}\,p{H2}^3}

Which species appear in Kp for a heterogeneous equilibrium?

Only the gases; solids are left out.

When does Kc have no units?

When there are the same number of concentration terms on the top and bottom lines of the expression.

How do you find equilibrium amounts from initial amounts?

Subtract the amount reacted from each reactant, add the amount formed to each product, using the mole ratio in the equation.

What must you do to amounts in moles before using Kc?

Divide by the volume in dm³ to get concentrations (unless the volume cancels).

What are the steps to calculate Kp from experimental data?

Equilibrium amounts, total moles, mole fractions, partial pressures, substitute into Kp, give units.

Kp and calculating equilibrium constantsEdexcel A-Level Chemistry: Flashcards

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Define partial pressure.

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Define partial pressure.: The pressure a gas in a mixture would exert if it alone occupied the container.
How do you calculate the partial pressure of a gas?: Mole fraction of the gas × total pressure.
How do you calculate a mole fraction?: Moles of the gas ÷ total moles of gas in the mixture.
What do the partial pressures in a mixture add up to?: The total pressure.
Write Kp for $\text{N}_2(g) + 3\text{H}_2(g) \rightleftharpoons 2\text{NH}_3(g)$ .: $K_p = \frac{p_{\text{NH}_3}^2}{p_{\text{N}_2}\,p_{\text{H}_2}^3}$
Which species appear in Kp for a heterogeneous equilibrium?: Only the gases; solids are left out.
What is the unit of Kp for $\text{N}_2\text{O}_4(g) \rightleftharpoons 2\text{NO}_2(g)$ with pressures in atm?: atm
What is the unit of Kp for $\text{N}_2 + 3\text{H}_2 \rightleftharpoons 2\text{NH}_3$ with pressures in atm?: atm⁻²
When does Kc have no units?: When there are the same number of concentration terms on the top and bottom lines of the expression.
How do you find equilibrium amounts from initial amounts?: Subtract the amount reacted from each reactant, add the amount formed to each product, using the mole ratio in the equation.
What must you do to amounts in moles before using Kc?: Divide by the volume in dm³ to get concentrations (unless the volume cancels).
What are the steps to calculate Kp from experimental data?: Equilibrium amounts, total moles, mole fractions, partial pressures, substitute into Kp, give units.

Exam questions on Kp and calculating equilibrium constants

Dinitrogen tetroxide dissociates reversibly: N₂O₄(g) ⇌ 2NO₂(g). In an experiment at 350 K the equilibrium mixture contained 0.40 mol of N₂O₄ and 0.20 mol of NO₂, and the total pressure was 3.0 atm.
Calculate the partial pressure of N₂O₄ and the value of Kp for this equilibrium, including its units.2 marks
In a sealed 2.00 dm³ vessel at 700 K, 1.00 mol of hydrogen and 1.00 mol of iodine were mixed and allowed to reach equilibrium: H₂(g) + I₂(g) ⇌ 2HI(g). At equilibrium the vessel contained 0.220 mol of hydrogen.
Calculate the equilibrium concentration of hydrogen iodide.2 marks
A mixture of 0.800 mol of sulfur dioxide and 0.600 mol of oxygen was sealed in a 2.00 dm³ vessel at 1000 K and allowed to reach equilibrium: 2SO₂(g) + O₂(g) ⇌ 2SO₃(g). At equilibrium the vessel contained 0.600 mol of sulfur trioxide.
Calculate the amounts of SO₂ and O₂ at equilibrium and the equilibrium concentration of each of the three gases.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).