Chemical Changes (College Board AP® Chemistry): Exam Questions

A student is given the task of determining the I⁻ content of tablets that contain KI and an inert, water-soluble sugar as a filler. A tablet is dissolved in 50.0 mL of distilled water, and an excess of 0.20 M Pb(NO₃)₂ (aq) is added to the solution. A yellow precipitate forms, which is then filtered, washed, and dried. The data from the experiment are shown in the table above.

For the chemical reaction that occurs when the precipitate forms,

i) write a balanced, net-ionic equation for the reaction, and

ii) explain why the reaction is best represented by a net-ionic equation. 

Explain the purpose of drying and weighing the filter paper with the precipitate three times.

In the filtrate solution, is [K⁺] greater than, less than, or equal to [NO₃⁻] ? Justify your answer.

Calculate the number of moles of precipitate that is produced in the experiment.

Calculate the mass percent of I⁻ in the tablet.

In another trial, the student dissolves a tablet in 55.0 mL of water instead of 50.0 mL of water. Predict whether the experimentally determined mass percent of I⁻ will be greater than, less than, or equal to the amount calculated in part (e). Justify your answer.

A student in another lab also wants to determine the I⁻ content of a KI tablet but does not have access to Pb(NO₃)₂ . However, the student does have access to 0.20 M AgNO₃ , which reacts with I⁻(aq) to produce AgI (s). The value of K_sp for AgI is 8.5 × 10⁻¹⁷.

i) Will the substitution of AgNO₃ for Pb(NO₃)₂ result in the precipitation of the I⁻ ion from solution? Justify your answer.

ii) The student only has access to one KI tablet and a balance that can measure to the nearest 0.01 g. Will the student be able to determine the mass of AgI produced to three significant figures? Justify your answer.