Name two sources of error in the lab on the solubility of lead iodine. Please note that human error or “bad instruments” are not valid sources of error. Describe what value the errors you identified affect.
The correct answer and explanation is:
Two valid sources of error in the lab on the solubility of lead iodide (PbI₂) that are not due to human mistakes or faulty instruments are:
1. Temperature Variations
Solubility of lead iodide is highly dependent on temperature. Even slight fluctuations in the temperature of the solution during the experiment can significantly affect the amount of PbI₂ that dissolves in water. If the solution is warmer than intended, more PbI₂ will dissolve, leading to an overestimation of its solubility. Conversely, if the temperature is lower, solubility will be underestimated. This error specifically affects the measured solubility value (g/100 mL water).
2. Incomplete Precipitation or Filtration
After forming the PbI₂ precipitate, if the solution is not allowed enough time for the solid to fully settle, or if the filtration process is incomplete, some undissolved PbI₂ may remain suspended in the filtrate or pass through the filter. This could cause the filtrate to appear as though more Pb²⁺ or I⁻ ions are still dissolved in the solution than actually are. As a result, the concentration of dissolved PbI₂ would be overestimated, impacting the final calculated solubility.
Explanation (300 words):
Understanding the solubility of lead iodide involves accurately determining how much of the compound dissolves in a given volume of water at a specific temperature. In this type of laboratory experiment, it is crucial to identify valid sources of systematic error that influence data collection and interpretation.
Temperature control is vital because solubility is a temperature-sensitive property. According to Le Chatelier’s Principle, an increase in temperature generally increases the solubility of solids. Therefore, even a small deviation from the target temperature (e.g., room temperature or 25°C) can yield solubility values that do not represent standard conditions, leading to skewed data.
Another significant factor is the incomplete separation of the precipitate. Lead iodide is a yellow solid that can be slow to settle due to its fine particle size. If the precipitate isn’t fully removed before measuring the concentration of ions in the filtrate, this misrepresents how much solute remains dissolved, thus altering the calculated solubility. Additionally, PbI₂ has low solubility, so even small amounts of retained solid can significantly affect the final result.
By controlling these two sources of error—maintaining constant temperature and ensuring thorough precipitation/filtration—the accuracy of solubility measurements in the lab can be greatly improved.