Calculating the Equilibrium Constant Practice

Sample questions

5 of 6 — sign in to practice the rest with adaptive difficulty and mastery tracking.

1. Sample 1difficulty 2/5

   A student determines K for Fe^3+ + SCN- <-> FeSCN^2+ by spectrophotometry at 447 nm. A standard curve made with [SCN-] in large excess gives epsilon = 4630 M^-1 cm^-1 (1.00 cm path). In the equilibrium trial, the student mixes equal volumes to give initial [Fe^3+] = 2.00e-3 M and initial [SCN-] = 2.00e-3 M (after dilution). The measured equilibrium absorbance is A = 1.85.

   What is the equilibrium concentration of FeSCN^2+?

   • A

     2.3e-7 M

   • B

     1.0e-3 M

   • C

     1.85e-3 M

   • D

     4.0e-4 M

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   Why

   [FeSCN^2+] = A / (epsilon * b) = 1.85 / (4630 * 1.00) = 4.0e-4 M.

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2. Sample 2difficulty 2/5

   A student determines K for Fe^3+ + SCN- <-> FeSCN^2+ by spectrophotometry at 447 nm. A standard curve made with [SCN-] in large excess gives epsilon = 4630 M^-1 cm^-1 (1.00 cm path). In the equilibrium trial, the student mixes equal volumes to give initial [Fe^3+] = 2.00e-3 M and initial [SCN-] = 2.00e-3 M (after dilution). The measured equilibrium absorbance is A = 1.85.

   What is the equilibrium constant K for Fe^3+ + SCN- <-> FeSCN^2+?

   • A

     4.0e-4

   • B

     200

   • C

     8.0e-7

   • D

     1.6e2

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   Why

   [FeSCN^2+]_eq = 4.0e-4 M. [Fe^3+]_eq = [SCN-]_eq = 2.00e-3 - 4.0e-4 = 1.6e-3 M each. K = 4.0e-4 / (1.6e-3)^2 = 4.0e-4/2.56e-6 = 156 ~ 1.6e2.

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3. Sample 3difficulty 3/5

   The reaction H₂(g) + I₂(g) ⇌ 2 HI(g) reaches equilibrium with the listed concentrations.

   What is Kc?

   • A

     8.0

   • B

     40

   • C

     0.016

   • D

     64

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   Why

   Kc = [HI]² / ([H₂][I₂]) = (1.60)² / [(0.20)(0.20)] = 2.56 / 0.04 = 64.

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4. Sample 4difficulty 3/5

   For 2 SO₂(g) + O₂(g) ⇌ 2 SO₃(g), equilibrium partial pressures are measured.

   Calculate Kp.

   • A

     0.025

   • B

     40

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   • C

     20

   • D

     10

   Why

   Kp = P_SO₃² / (P_SO₂² · P_O₂) = (0.40)² / [(0.20)²(0.10)] = 0.16 / 0.004 = 40.

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5. Sample 5difficulty 3/5

   For A ⇌ B + C: [A] = 0.20, [B] = 0.10, [C] = 0.10. K =

   • A

     0.20

   • B

     0.10

   • C

     0.05

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   • D

     0.50

   Why

   K = [B][C]/[A] = 0.10·0.10/0.20 = 0.05.

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