Question

What is the minimum number of moles of Pb(NO3) must be added to 0.10L of a solution that is 1.0M in MgCl2 and 1.0 M in KCl? The compound PbCl2 precipitates.

• A. 1.0 mol
• B. 0.20 mol
• C. 0.50 mol
• D. 0.15 mol

Hint:

When calculating the amount of substance needed to precipitate a compound, consider the concentrations of the ions involved and use the solubility product constant (Ksp).

Answer 1

The Correct Option is D

The precipitation of PbCl2 occurs when the concentration of chloride ions (Cl⁻) exceeds the solubility product (Ksp) of PbCl2. In this case, we are given 1.0 M MgCl2 and 1.0 M KCl, which both dissociate to release chloride ions. The total concentration of chloride ions in the solution is the sum of the chloride ions from both compounds, which is 2.0 M.

To precipitate PbCl2, the concentration of Pb²⁺ must reach a level where the product of the concentrations of Pb²⁺ and Cl⁻ exceeds the Ksp of PbCl2. This condition causes the PbCl2 to precipitate out of solution. Using the Ksp value for PbCl2 and the chloride ion concentration, we can calculate the minimum concentration of Pb²⁺ required to exceed the Ksp and initiate precipitation.

Finally, to calculate the minimum moles of Pb(NO3)2 required to achieve this concentration of Pb²⁺, we find that the minimum moles of Pb(NO3)2 needed to form PbCl2 is 0.15 mol.

Therefore, the minimum moles of Pb(NO3)2 required to form PbCl2 are 0.15 mol.