What is Quantitative Analysis and Gravimetry?
Quantitative analysis determines the exact amount or concentration of a substance in a sample. Gravimetry is a key quantitative method that measures mass changes to deduce the identity and amount of chemical substances.
Gravimetric analysis isolates a target ion as an insoluble compound, filters, dries and weighs it to find the original amount. The mass of the precipitate reveals how much of the analyte was present.
- 1↓Sample PreparationDissolve or suspend the sample in suitable solvent
- 2↓PrecipitationAdd reagent to form insoluble compound
- 3↓FiltrationFilter the precipitate using Buchner funnel
- 4↓DryingHeat in oven to remove water (105–110 °C)
- 5↓WeighingCool in desiccator, weigh on analytical balance
- 6CalculationUse stoichiometry to find original analyte mass
Step-by-step worked examples
A 0.5 g sample containing BaCl₂ is precipitated as BaSO₄ (Molar mass = 233 g/mol). The dried precipitate weighs 0.233 g. Find the % BaCl₂ in the sample.
Moles of BaSO₄ = 0.233 g / 233 g/mol = 0.001 mol From stoichiometry: 1 mol Ba²⁺ → 1 mol BaSO₄ Moles of Ba²⁺ = 0.001 mol Moles of BaCl₂ = 0.001 mol Mass of BaCl₂ = 0.001 × 208 = 0.208 g % BaCl₂ = (0.208 / 0.5) × 100 = 41.6%
Silver is determined gravimetrically by precipitating as AgCl. If 1.0 g of sample gives 0.144 g AgCl precipitate (Molar mass AgCl = 144 g/mol), what is the mass of Ag?
Moles of AgCl = 0.144 g / 144 g/mol = 0.001 mol From equation: 1 mol Ag⁺ → 1 mol AgCl Moles of Ag⁺ = 0.001 mol Mass of Ag = 0.001 × 108 = 0.108 g
A limestone sample (0.8 g) is dissolved and CO₃²⁻ is precipitated as CaCO₃ (Molar mass = 100 g/mol). Precipitate mass = 0.160 g. Find % CaCO₃.
Moles of CaCO₃ = 0.160 / 100 = 0.0016 mol Mass of CaCO₃ = 0.0016 × 100 = 0.160 g % CaCO₃ = (0.160 / 0.8) × 100 = 20%
Flashcards
Quick quiz
Q1.In gravimetric analysis, why is the precipitate heated?
Q2.If you weigh the precipitate while still hot, what error occurs?
Q3.Gravimetry assumes the precipitate is…
Q4.What is 'constant mass' in gravimetry?
The full card deck, worked steps and AI-tutor support for “What is Quantitative Analysis and Gravimetry?” are in Notek — study by hand before your exam.
Common mistakes
Weighing the wet or hot precipitate directly. — Correct: Always dry at specified temperature and cool in desiccator before weighing.
Using the mass of solution instead of the dry residue. — Correct: The mass of the solid precipitate alone determines the analyte content.
Forgetting to convert moles of precipitate to moles of analyte. — Correct: Use stoichiometry — the mole ratio depends on the balanced chemical equation.
Assuming the precipitate is completely separated after one filtration. — Correct: Small amounts may remain in solution; rinse the filter multiple times for quantitative transfer.
FAQ
What is quantitative analysis in chemistry?
Determining the exact amount or percentage of a substance in a sample using precise measurements.
How does gravimetric analysis work?
It isolates the analyte as an insoluble precipitate, filters and dries it, then weighs it to deduce the original amount.
What is the purpose of the desiccator?
To cool the hot precipitate without it reabsorbing moisture from the air before weighing.
What does 'constant mass' mean?
When repeated heating and weighing cycles give masses differing by <0.5 mg, confirming all water is removed.