**Experiment 1** Students performed a gravimetric analysis to determine the concentration … — ACT

Experiment 1
Students performed a gravimetric analysis to determine the concentration of sulfate ions ( $\text{SO}_4^{2-}$ ) in a water sample using a precipitation and vacuum filtration apparatus.

Arrange the following experimental steps in the correct procedural order from first to last to ensure complete precipitation, crystal growth, isolation, and mass determination of the resulting barium sulfate ( $\text{BaSO}_4$ ).

1Acidify the aqueous water sample with dilute hydrochloric acid ( $\text{HCl}$ ) and heat the solution to near boiling.
2Add hot barium chloride ( $\text{BaCl}_2$ ) solution dropwise while stirring to precipitate barium sulfate ( $\text{BaSO}_4$ ).
3Digest the precipitate mixture by holding it just below boiling temperature for 1 hour.
4Filter the digested mixture through a pre-weighed Gooch crucible attached to a vacuum filtration suction flask.
5Heat the Gooch crucible with precipitate in a muffle furnace at $800^\circ\text{C}$ , cool in a desiccator, and record the final mass.

Answer

The correct procedural sequence is: 1) Acidify the aqueous sample with dilute

\text{HCl}

and heat near boiling; 2) Add hot

\text{BaCl}_2

dropwise while stirring; 3) Digest the precipitate mixture just below boiling for 1 hour; 4) Filter through a pre-weighed Gooch crucible using vacuum filtration; 5) Heat the crucible in a muffle furnace at

800^\circ\text{C}

, cool in a desiccator, and record final mass.

The correct sequence follows standard quantitative gravimetric procedures: sample solution preparation (acidifying and heating), controlled precipitation (dropwise addition of reagent), digestion (crystal growth), filtration (vacuum suction separation), and thermal ignition/desiccation (moisture removal and final weighing).

Step-by-Step Solution

Prepare the solution environment before introducing the precipitating agent.

Sample solution is acidified with

\text{HCl}

and heated.

Acidification prevents interfering side reactions (such as barium carbonate formation), and heating sets up the system for controlled precipitation.

Introduce the precipitating reagent under controlled conditions.

Slow dropwise addition of

\text{BaCl}_2

produces

\text{BaSO}_4

precipitate.

Slow, dropwise addition while stirring prevents supersaturation and minimizes occlusion of impurities.

Digest the precipitate prior to filtration.

Precipitate crystals grow larger (Ostwald ripening).

Smaller crystals dissolve and deposit onto larger crystals, preventing fine particles from clogging or passing through the filter medium.

Separate the solid precipitate from the liquid phase using the apparatus.

Solid

\text{BaSO}_4

is collected on a pre-weighed Gooch crucible via vacuum filtration.

The solution must be drawn through the crucible to isolate the solid before final mass measurement.

Thermal treatment and gravimetric measurement.

Dry, pure

\text{BaSO}_4

mass is obtained after furnace ignition and cooling in a desiccator.

High-temperature ignition burns off leftover moisture, and cooling in a desiccator prevents atmospheric re-absorption of water prior to weighing.

Key Concept

Gravimetric Analysis and Procedural Order in Precipitation Apparatus

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