A chemist precipitates chloride ions as AgCl and weighs the dried precipitate. The result implies a sulfate content 8% higher than expected from an independent analysis. Which explanation best accounts for this positive error?
AKsp of AgCl was too high, so excess chloride remained in solution — a negative error source, not positive
BCoprecipitation — foreign ions were adsorbed onto or occluded within the AgCl crystals, adding mass beyond the analyte's contribution
CThe gravimetric factor was applied upside down, inflating the calculated analyte mass
DThe precipitate was dried at too low a temperature, retaining water
Coprecipitation introduces foreign mass into the precipitate, directly causing positive errors — the precipitate weighs more than pure analyte-derived compound, so the calculated analyte is too high. This typically occurs when precipitation is rapid from concentrated solution, generating many tiny crystals with high surface area that trap impurities via adsorption or occlusion. The remedy is slow precipitation from hot, dilute solution to grow larger, purer crystals. Note: the question says 'sulfate content' — this is likely a mislabeled analyte in a real scenario; the error mechanism is coprecipitation regardless.
Question 2 Multiple Choice
Why is gravimetric analysis used to establish primary standards, while UV-Vis spectrophotometry requires calibration curves?
AGravimetric instruments are more expensive and therefore more accurate
BGravimetry is more sensitive than spectrophotometric methods at low concentrations
CGravimetry measures mass — an absolute, fundamental SI unit — requiring no comparison to external standards at measurement time; spectrophotometry measures relative absorbance, which drifts and must be calibrated
DGravimetry works on more analyte types and is therefore more versatile
The power of gravimetry lies in using mass as its signal. Mass is traceable directly to fundamental SI units (the kilogram) without requiring instrument calibration or reference standards at the time of measurement. An analytical balance simply weighs; it does not need to be 'taught' what a known standard looks like. Spectrophotometry measures absorbance, which is an instrument response that depends on lamp intensity, detector sensitivity, and cell path length — all of which drift and must be corrected via calibration curves using known standards. This is why gravimetry establishes the primary standards against which spectrophotometric and other methods are validated.
Question 3 True / False
Gravimetric accuracy requires that the Ksp of the precipitate be very small (typically below 10⁻⁸) to ensure nearly complete precipitation of the analyte.
TTrue
FFalse
Answer: True
If Ksp is too large, a measurable quantity of the analyte remains dissolved in the supernatant rather than transferring to the precipitate — producing a negative error (precipitate weighs less than expected → calculated analyte is too low). For quantitative precipitation (≥99.9% capture), Ksp must be very small. Adding excess precipitating reagent exploits the common ion effect to further suppress solubility, driving precipitation toward completion.
Question 4 True / False
Coprecipitation generally causes a positive error in gravimetric analysis, making the calculated analyte mass too high.
TTrue
FFalse
Answer: False
Coprecipitation can cause either positive or negative errors, depending on the nature of the contaminant. If foreign ions are simply adsorbed onto the precipitate surface (adding extra mass) or occluded within the lattice, the result is a positive error. But if the coprecipitated impurity replaces analyte ions with lighter species in the crystal structure, the precipitate may weigh less per mole of analyte than expected, producing a negative error. The direction depends on the chemical identity and mass of the contaminating species relative to the analyte ion.
Question 5 Short Answer
Why is slow precipitation from hot, dilute solution preferred in gravimetric analysis, even though it is more time-consuming than rapid precipitation?
Think about your answer, then reveal below.
Model answer: Slow precipitation from hot, dilute solution produces fewer nuclei and allows existing crystals to grow large. Large crystals have proportionally less surface area than many small ones, which reduces coprecipitation (fewer sites for impurity adsorption) and makes filtration more efficient (less loss through filter paper). The von Weimarn ratio shows that high supersaturation — from rapid mixing of concentrated solutions — nucleates a burst of tiny crystallites with enormous surface area that readily trap impurities. Heat also increases solubility temporarily, reducing supersaturation and further favoring crystal growth over nucleation.
In practice: after adding the precipitating reagent, the solution is digested (kept warm for 30–60 minutes) to allow small crystals to dissolve and redeposit on larger ones — Ostwald ripening. The result is a coarser, purer precipitate that filters cleanly and gives more accurate results.