[VISUAL] 1. A student is given the task of determining the I− content of tablets that contain KI and — Stoichiometry Chemistry Question
Problem Context
[VISUAL]
- 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.
Model Answer
Pb2+ + 2 I− → PbI2
1 point is earned for a balanced net-ionic equation. || The net-ionic equation shows the formation of the PbI2(s) from Pb2+(aq) and I−(aq) ions, omitting the non-reacting species (spectator ions), K+(aq) and NO3−(aq).
1 point is earned for a valid explanation.
Model Answer
The filter paper and precipitate must be dried several times (to a constant mass) to ensure that all the water has been driven off.
1 point is earned for a valid explanation.
Model Answer
[K+] is less than [NO3 −] because the source of the NO3 −, the 0.20 M Pb(NO3)2(aq), was added in excess.
1 point is earned for a correct comparison with a valid explanation.
Model Answer
1.698 g − 1.462 g = 0.236 g PbI2(s)
0.236 g PbI2 × (1 mol PbI2 / 461.0 g PbI2) = 5.12 × 10^-4 mol PbI2
1 point is earned for the correct number of moles of PbI2(s) precipitate.
Model Answer
5.12 × 10^-4 mol PbI2 × (2 mol I- / 1 mol PbI2) = 1.02 × 10^-3 mol I-
1.02 × 10^-3 mol I- × (126.91 g I- / 1 mol I-) = 0.130 g I- in one tablet
0.130 g I- / 0.425 g KI tablet = 0.306 = 30.6% I- per KI tablet
1 point is earned for determining the number of moles of I− in one tablet.
1 point is earned for calculating the mass percent of I− in the KI tablet.
Model Answer
The mass percent of I− will be the same. Pb2+(aq) was added in excess, ensuring that essentially no I− remained in solution. The additional water is removed by filtration and drying, leaving the same mass of dried precipitate.
1 point is earned for correct comparison with a valid justification.
Model Answer
Yes. Addition of an excess of 0.20 M AgNO3(aq) will precipitate all of the I− ion present in the solution because AgI is insoluble, as evidenced by its low value of Ksp.
1 point is earned for the correct answer with a valid justification. || No. If masses can be measured to ±0.01 g, then the mass of the dry AgI(s) precipitate (which is less than 1 g) will be known to only two significant figures.
1 point is earned for a correct answer with a valid justification.