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Difficulty: MediumAnalyzing Experimental Procedures and Apparatus

Experiment 1
Students conducted a gravimetric analysis to determine the phosphate concentration in a water sample by precipitating magnesium ammonium phosphate hexahydrate (MgNH4PO4⋅6H2O\text{MgNH}_4\text{PO}_4 \cdot 6\text{H}_2\text{O}). The apparatus included a Büchner funnel, a heavy-walled filter flask attached to a vacuum aspirator line, filter paper, a wash bottle filled with ice-cold dilute ammonia solution, and an oven.

To ensure quantitative transfer, efficient impurity removal, and accurate final mass measurement of the precipitate without losing product, in what chronological order should the students perform the following steps of the filtration and drying procedure?

  1. 1Place the filter paper in the Büchner funnel, turn on the vacuum aspirator, and moisten the filter paper with a small amount of solvent to seat it flush against the perforated plate.
  2. 2Pour the reaction mixture slurry into the center of the funnel, allowing the liquid filtrate to be drawn into the vacuum flask while solid precipitate accumulates on the paper.
  3. 3Rinse the solid filter cake with small portions of ice-cold dilute ammonia solution under vacuum suction to clear residual dissolved ions from the precipitate.
  4. 4Draw air through the filter cake under vacuum to pull out excess moisture, then transfer the filter paper with precipitate into a drying oven to reach constant mass.

Answer

The correct sequence is to first seat and moisten the filter paper under vacuum, second pour the reaction mixture slurry to collect the precipitate, third rinse the filter cake with ice-cold wash solution, and fourth air-dry under vacuum before heating in a drying oven to constant mass.
The correct order follows standard laboratory protocol for gravimetric vacuum filtration: seating the filter paper prevents solids from leaking past the filter edges; pouring collects the bulk solid; washing cleans residual dissolved ions from the solid filter cake; and final oven drying removes moisture to allow accurate mass measurement.

Step-by-Step Solution

1
Identify the initial setup requirement for vacuum filtration.
Moistening and vacuum-seating the filter paper must occur first.
If the paper is not seated tightly with liquid prior to adding the slurry, solid precipitate can leak underneath the paper edges and be lost in the filtrate.
2
Determine the primary separation step.
Pouring the reaction mixture slurry onto the prepared funnel.
Filtration separates the solid precipitate cake from the liquid supernatant.
3
Identify the purification step.
Washing the collected solid with ice-cold dilute ammonia.
Washing removes soluble surface impurities clinging to the solid cake; doing this after pouring ensures maximum liquid-solid contact.
4
Determine the final isolation and drying step.
Air-drying under vacuum followed by drying in an oven to constant mass.
Drying must occur last because any prior step involving washing or liquid transfer would reintroduce moisture, invalidating mass measurements.

Key Concept

Chronological execution and function of vacuum filtration apparatus steps
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Topics

Analyzing Experimental Procedures and Apparatus

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