Relating Kp and Kc: Class 11 practice
For gaseous equilibria Kp = Kc(RT)^Δn, where Δn = moles of gaseous products − moles of gaseous reactants and R = 0.0821 L atm mol⁻¹ K⁻¹. Read the method, follow the three worked examples, then try the six questions.
What you should know first
- Equilibrium constant
The idea behind relating kp and kc
For gaseous equilibria Kp = Kc(RT)^Δn, where Δn = moles of gaseous products − moles of gaseous reactants and R = 0.0821 L atm mol⁻¹ K⁻¹. If Δn = 0, Kp = Kc.
Step-by-step method
- Count gaseous moles on each side.
- Δn = products − reactants.
- Kp = Kc(RT)^Δn.
Worked example 1 (Basic)
Find Δn for CaCO₃(s) ⇌ CaO(s) + CO₂.
- Gaseous products − gaseous reactants
- Answer: 1
Worked example 2 (Standard)
For H₂ + I₂ ⇌ 2HI, Kc = 4.6 at 352 K. Find Kp.
- Δn = 0
- Kp = 4.6 × (0.0821 × 352)^0
- Answer: 4.600
Worked example 3 (Challenge)
For CaCO₃(s) ⇌ CaO(s) + CO₂, Kc = 1.7 at 616 K. Find Kp.
- Δn = 1
- Kp = 1.7 × (0.0821 × 616)^1
- Answer: 85.98
Common mistakes and misconceptions
- Counting solids or liquids in Δn
- Sign of Δn
Practice questions
- (Basic) Find Δn for 2SO₂ + O₂ ⇌ 2SO₃.
- (Basic) Find Δn for H₂ + I₂ ⇌ 2HI.
- (Standard) For N₂ + 3H₂ ⇌ 2NH₃, Kc = 0.9 at 464 K. Find Kp.
- (Standard) For N₂ + 3H₂ ⇌ 2NH₃, Kc = 1.5 at 621 K. Find Kp.
- (Challenge) For N₂ + 3H₂ ⇌ 2NH₃, Kc = 3.2 at 546 K. Find Kp.
- (Challenge) For CaCO₃(s) ⇌ CaO(s) + CO₂, Kc = 0.4 at 799 K. Find Kp.
Answer key with full working
- Q1: Gaseous products − gaseous reactants → Answer: -1
- Q2: Gaseous products − gaseous reactants → Answer: 0
- Q3: Δn = -2 → Kp = 0.9 × (0.0821 × 464)^-2 → Answer: 0.0006202
- Q4: Δn = -2 → Kp = 1.5 × (0.0821 × 621)^-2 → Answer: 0.0005771
- Q5: Δn = -2 → Kp = 3.2 × (0.0821 × 546)^-2 → Answer: 0.001592
- Q6: Δn = 1 → Kp = 0.4 × (0.0821 × 799)^1 → Answer: 26.24
Syllabus note
This skill is listed under "Equilibrium" for Class 11 in the NCERT/CBSE books. ICSE, IGCSE, IB and state boards may teach it in a different chapter or year.
Answers on this page are generated and automatically re-checked by a second, independent calculation. They have not been reviewed by a teacher — if you spot a problem, tell us on WhatsApp.
Related practice and notes
Frequently asked questions
Which chapter is relating kp and kc in?
"Equilibrium", Class 11 Chemistry (NCERT/CBSE). Other boards may place it differently.
How were the answers checked?
Answers on this page are generated and automatically re-checked by a second, independent calculation. They have not been reviewed by a teacher — if you spot a problem, tell us on WhatsApp.