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Which of the following substances will DECREASE the solubility of Pb(OH)₂ in saturated solution?

a) NaNO₃
b) HNO₃
c) Pb(NO₃)₂
d) HCl
e) NaCl

1 Answer

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Final answer:

HNO₃ decreases the solubility of Pb(OH)₂ in a saturated solution by providing additional H⁺ ions that react with OH⁻ ions to form water, illustrating the common ion effect and causing the equilibrium to shift and precipitate more Pb(OH)₂.

Step-by-step explanation:

Among the given substances, HNO₃ will decrease the solubility of Pb(OH)₂ in a saturated solution. This is because HNO₃ is a strong acid and will provide additional H⁺ ions when it dissociates in water. The added H⁺ ions will react with the hydroxide ions (OH⁻) from the saturated Pb(OH)₂ solution, forming water and thereby reducing the concentration of OH⁻ ions in solution.

The reaction can be represented as follows:

  • HNO₃ (aq) → H⁺ (aq) + NO₃⁻ (aq)
  • H⁺ (aq) + OH⁻ (aq) → H₂O (l)

This process is a classic example of the common ion effect, where adding an ion that is already present in the solution causes the equilibrium to shift, in this case, by precipitating more Pb(OH)₂, thus decreasing its solubility.

The other options such as NaNO₃, Pb(NO₃)₂, NaCl, and HCl do not decrease the solubility of Pb(OH)₂ in the same way. NaNO₃ and Pb(NO₃)₂ provide NO₃⁻ ions that do not affect the equilibrium significantly, as both Pb²⁺ and OH⁻ are still present. NaCl provides Na⁺ and Cl⁻ ions, which do not directly affect the dissolution equilibrium of Pb(OH)₂. While HCl also provides H⁺ ions, it also introduces Cl⁻ ions which will form a soluble complex with Pb²⁺, so its effect on Pb(OH)₂ solubility depends on the exact conditions of the reaction.

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