Investigation 3: Gravimetric Analysis

If an ion in solution has a partner that forms an insoluble compound, you can pull it out of the water entirely. Add an excess of that partner ion, and every bit of the target ion locks into solid precipitate. Nothing of it is left dissolved. That means the mass of solid you collect is directly tied, through the precipitate's known formula and molar mass, to the amount of the original ion. Weighing a solid is far more precise than almost any other way of measuring a dissolved quantity, which is why this method is trusted for exact concentrations.

The procedure has four stages. First, precipitate: add excess precipitating reagent so the target ion is completely consumed. Second, filter: pour the mixture through filter paper to physically trap the solid while the liquid and dissolved leftover ions pass through. Third, dry: heat the filter paper and solid gently, usually in an oven, to drive off every trace of water and solvent. Fourth, weigh to constant mass: weigh, reheat, and reweigh repeatedly until two consecutive masses agree. That agreement is the only proof that drying is finished and the sample is pure solid.

This investigation is where the error-analysis and particulate-drawing practices from this unit meet real numbers. A damp or incompletely filtered sample produces a mass that does not represent the actual compound, so every mole and concentration calculated from it is wrong in a predictable direction. Thinking through which direction the error pushes the final answer is the same reasoning skill used across the unit's other quantitative labs.