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Question:

An aqueous solution 0.10M H₂S and 0.20M HCl. If the equilibrium constants for the formation of HS⁻ from H₂S is 1.0 × 10⁻⁷ and that of S²⁻ from HS⁻ ions is 1.2 × 10⁻¹³, then the concentration of S²⁻ ions in aqueous solution is:

5 × 10⁻¹⁴

6 × 10⁻¹⁵

5 × 10⁻¹⁶

3 × 10⁻¹⁶

Solution:

H₂S⇌ 2H⁺ + S²⁻

0.1 - α 0.2 + 2α α

Keq = K₁ × K₂ = 1.2 × 10⁻²⁰

1.2 × 10⁻²⁰ = (0.2 + 2α)²(α) / (0.1 - α)

As Keq is too small, α <<< 0.1

Therefore, 0.2 + 2α ≈ 0.2

0.2² × α = 1.2 × 10⁻²⁰

α = 3 × 10⁻¹⁶