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Equilibrium appeared 36 times across 3 years — 4.2% of Chemistry. This question is from Chemical Equilibrium and Le Chatelier's Principle.

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X₂(g) + Y₂(g) leftharpoons 2Z(g) X₂(g) and Y₂(g) are added to a 1 L flask and it is found that the system attains the above equilibrium at T(K) with the number of moles of X₂(g), Y₂(g) and Z(g) being 3, 3 and 9 mol respectively (equilibrium moles). Under this conditions of equilibrium, 10 mol of Z(g) is added to the flask and the temperature is maintained at T(K). Then the number of moles of Z(g) in the flask when the new equilibrium is established is ____. (Nearest integer).

Numerical Answer Type:
Enter a numerical value Answer: 15 to 15 +4 marks

Solution & Explanation

Related Formula
Kc = ([Z]²)/([X₂][Y₂])
Core Logic

For the reaction X₂(g) + Y₂(g) leftharpoons 2Z(g): First, calculate the equilibrium constant Kc using the initial equilibrium concentrations (moles in 1 L volume). [X₂] = 3 M [Y₂] = 3 M [Z] = 9 M

Kc = ((9)²)/((3)(3)) = (81)/(9) = 9
Step 1: Shift due to addition of Z

Now, 10 moles of Z(g) are added. The new initial moles are: X₂ = 3 Y₂ = 3 Z = 9 + 10 = 19

The reaction quotient Qc > Kc, so the reaction shifts backward. Let the change in X₂ and Y₂ be +x moles, then Z changes by -2x moles. At new equilibrium: X₂ = 3 + x Y₂ = 3 + x Z = 19 - 2x

Step 2: Calculate New Equilibrium
Kc = ((19 - 2x)²)/((3 + x)(3 + x)) = 9

Take the square root of both sides (concentrations must be positive):

(19 - 2x)/(3 + x) = 3 19 - 2x = 3(3 + x) 19 - 2x = 9 + 3x 10 = 5x x = 2

The number of moles of Z(g) at the new equilibrium is:

Moles of Z = 19 - 2x = 19 - 2(2) = 19 - 4 = 15
Pattern Recognition

If a reaction's Kc evaluates to a perfect square and Δ ng = 0, adding products sets up a neat quadratic that can be directly resolved by taking the root of both sides, heavily speeding up the algebraic work.

Chapter Mix

Class 11 Chemistry: Equilibrium

Reference Study Guides

Equilibrium

More Equilibrium Previous-Year Questions — Page 8

Q62 jee_main_2024_31_jan_morning Equilibrium Constant
For the given reaction, choose the correct expression of KC from the following :- Fe(aq)³⁺ + SCN(aq)⁻ leftharpoons (FeSCN)(aq)²⁺
  • A. KC = [FeSCN²⁺][Fe³⁺][SCN⁻]
  • B. KC = [Fe³⁺][SCN⁻][FeSCN²⁺]
  • C. KC = [FeSCN²⁺][Fe³⁺]²[SCN⁻]²
  • D. KC = [FeSCN²⁺]²[Fe³⁺][SCN⁻]

Solution

Related Formula
KC = [Products][Reactants]
Core Logic
KC = Products ion conc.Reactants ion conc. KC = [FeSCN²⁺][Fe³⁺][SCN⁻]
Chapter Mix

Class 11 Chemistry: Equilibrium

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