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Lessons · Precipitation

Which reactant runs out when a precipitate forms?

Machine-checked — balanced, charge-conserved, extent-verified by ChemKernel Solubility rule-sourced (openstax-chemistry-2e) 3 modeling assumptions (disclosed)

25.0 mL of 0.100 M CaCl₂(aq) is mixed with 20.0 mL of 0.150 M Na₂CO₃(aq). A white precipitate forms. What mass of calcium carbonate forms, which reactant limits the reaction, and which ions remain in solution?

Mass of CaCO3\mathrm{CaCO_{3}} precipitate
0.250 g
Limiting reagent
CaCl₂
Left in solution
Na₂CO₃ 0.5 mmol
Molecular equation — what you combine
CaCl2 (aq)+Na2CO3 (aq)→CaCO3 (s)+2 NaCl (aq)\mathrm{CaCl_{2}}\,\text{(aq)} + \mathrm{Na_{2}CO_{3}}\,\text{(aq)} \rightarrow \mathrm{CaCO_{3}}\,\text{(s)} + 2\,\mathrm{NaCl}\,\text{(aq)}
Complete ionic — every strong electrolyte shown as free ions strong-electrolyte model
Ca2++2 Cl−+2 Na++CO32−→CaCO3+2 Na++2 Cl−\mathrm{Ca}^{2+} + 2\,\mathrm{Cl}^{-} + 2\,\mathrm{Na}^{+} + \mathrm{CO_{3}}^{2-} \rightarrow \mathrm{CaCO_{3}} + 2\,\mathrm{Na}^{+} + 2\,\mathrm{Cl}^{-}
Net ionic — spectators cancelled, the reaction that actually happens
Ca2++CO32−→CaCO3\mathrm{Ca}^{2+} + \mathrm{CO_{3}}^{2-} \rightarrow \mathrm{CaCO_{3}}

Spectator ions (unchanged, still dissolved): Cl⁻ Na⁺

Verification Every claim below was proven at build time — not asserted.
  • ✓ Atoms balance across the equation [conservation matrix]
  • ✓ Charge balances (net ionic re-verified) [charge row]
  • ✓ Units cancel through the dimensional chain [units engine]
  • ✓ No amount goes negative — extent is physical [nonnegative-extent guard]

rule-sourced CaCO₃ is treated as insoluble by rule insol-carbonate: “Carbonates are insoluble, except those of group 1 cations and ammonium.” (openstax-chemistry-2e).

✗ Common misconception: “The limiting reagent is whichever reactant has the smaller volume.”

The smaller-volume solution (Na₂CO₃(aq), 20.0 mL) is actually in excess. Moles decide it, not volume: CaCl₂ reaches 0 in the ledger and limits the reaction — watch the final (mol) column.

Modeling assumptions — author-asserted, disclosed not discharged
  • model CaCl₂ and Na₂CO₃ are strong electrolytes and dissociate completely in water.
  • rule CaCO₃ is treated as insoluble and precipitates completely.
  • model Solution volumes are additive.

Concepts in this lesson

Linked into the Chemical Atlas where an entry exists; the rest fill in as the Atlas grows.

Practice this

The lesson goes deep on one scenario; the gym builds fluency by repetition. Drill these: