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Ch.17 - Additional Aspects of Aqueous Equilibria
Brown - Chemistry: The Central Science 15th Edition
Brown15th EditionChemistry: The Central ScienceISBN: 9780137542970Not the one you use?Change textbook
All textbooksBrown 15th EditionCh.17 - Additional Aspects of Aqueous EquilibriaProblem 72
Chapter 17, Problem 72

Suppose that a 10-mL sample of a solution is to be tested for I- ion by addition of 1 drop (0.2 mL) of 0.10 M Pb1NO322. What is the minimum number of grams of I- that must be present for PbI21s2 to form?

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Step 1: Write down the balanced chemical equation for the reaction. The reaction between lead nitrate (Pb(NO3)2) and iodide ions (I-) forms lead iodide (PbI2) and nitrate ions (NO3-). The balanced chemical equation is: Pb(NO3)2(aq) + 2I-(aq) -> PbI2(s) + 2NO3-(aq)
Step 2: Calculate the moles of Pb(NO3)2 that are added to the solution. The molarity (M) is defined as the number of moles of solute per liter of solution. Therefore, the number of moles of Pb(NO3)2 can be calculated by multiplying the volume of the solution (in liters) by the molarity. In this case, the volume is 0.2 mL (or 0.0002 L) and the molarity is 0.10 M.
Step 3: Use the stoichiometry of the reaction to find the minimum number of moles of I- needed to react with the Pb(NO3)2. From the balanced chemical equation, we can see that one mole of Pb(NO3)2 reacts with two moles of I-. Therefore, the minimum number of moles of I- needed is twice the number of moles of Pb(NO3)2.
Step 4: Convert the moles of I- to grams. The molar mass of I- is approximately 126.9 g/mol. Therefore, the number of grams of I- can be calculated by multiplying the number of moles by the molar mass.
Step 5: The result from step 4 is the minimum number of grams of I- that must be present in the 10-mL sample for PbI2 to form.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Stoichiometry

Stoichiometry is the calculation of reactants and products in chemical reactions. It involves using balanced chemical equations to determine the proportions of substances involved. In this case, understanding the stoichiometric relationship between lead(II) ions (Pb²⁺) and iodide ions (I⁻) is essential to calculate the minimum amount of I⁻ needed to form lead(II) iodide (PbI₂).
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Molarity

Molarity is a measure of concentration defined as the number of moles of solute per liter of solution. It is crucial for determining how much of a substance is present in a given volume of solution. In this question, the molarity of the Pb(NO₃)₂ solution (0.10 M) will be used to find the number of moles of Pb²⁺ that can react with I⁻ ions.
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Precipitation Reaction

A precipitation reaction occurs when two soluble salts react to form an insoluble product, known as a precipitate. In this scenario, the formation of PbI₂ as a solid precipitate from the reaction of Pb²⁺ and I⁻ ions is the key outcome. Understanding the conditions under which this precipitate forms is vital for determining the minimum amount of I⁻ required.
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Related Practice
Textbook Question

A solution of Na2SO4 is added dropwise to a solution that is 0.010 M in Ba2+(aq) and 0.010 M in Sr2+(aq). (c) What is the concentration of SO42-(aq) when the second cation begins to precipitate?

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Textbook Question

A solution of Na2SO4 is added dropwise to a solution that is 0.010 M in Ba2+(aq) and 0.010 M in Sr2+(aq). (a) What concentration of SO42- is necessary to begin precipitation? (Neglect volume changes. BaSO4: Ksp = 1.1⨉10-10; SrSO4: Ksp = 3.2⨉10-7.)

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