Finding a reacting ratio from precipitate height sim
Fill eight tubes, add more and more of a second solution, let the precipitates settle, then measure them with a ruler at eye level. The corner of your graph gives the mole ratio and the equation. Dry mass gives the same corner.
Stoichiometry: reacting volumes, limiting reactants and deducing an equation from a precipitate-height practical · covers the GCSE (Higher), IGCSE (Supplement), O-Level topic
What the sim shows
When two solutions react to form a precipitate, the amount of solid grows as you add more of the second solution, until the first one is used up. After that, adding more makes no more solid: the second solution is in excess. Plot the height (or the dry mass) of the precipitate against the volume added, draw two straight lines, and read the corner. At the corner the two reactants have exactly reacted, so their mole ratio gives the equation. Each tube holds 5.0 cm³ of the first solution: copper(II) sulfate 0.20 mol/dm³, iron(III) chloride 0.10 mol/dm³ or barium chloride 0.40 mol/dm³; the second solution is 0.50 mol/dm³ (concentrations chosen so the corner falls between 1 and 8 cm³). Tests on the liquid above the solid show which ion is left over: before the corner, a drop of the second solution makes more precipitate (and copper(II) ions keep the liquid blue); after it, a drop of the first solution does, and leftover hydroxide turns damp red litmus blue. Height is only a rough measure of amount: a settled precipitate holds a lot of water and packs a little differently in every tube (here by up to 6 %), and it must be left to settle (here about 20 minutes). Read the ruler with your eye level with the top of the solid, or parallax makes the reading too high or too low. The dry mass is a better measure and gives the same corner; the solid is dried at room temperature (in a desiccator), because warming turns copper(II) hydroxide into black copper(II) oxide and changes the masses. Ar values from the data booklet. Model limits: the settled height per mmol, the settling time and the packing spread are illustrative; the close-up exaggerates the gap between the scale and the solid (12 mm, nearer 4 mm in a real tube), so real parallax is about a third as big; the tests are the textbook results; time moves only when you press Wait 5 minutes.
- Tubes 1–8: 5.0 cm³ of the first solution each
- Graduated pipette: 1.0 to 8.0 cm³ of the second
- Ruler held against a tube; eye level with the solid
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