Every time a water authority tests your tap water for mineral content, or a food manufacturer checks the salt level in a product, they are likely using gravimetric analysis. It is one of the oldest quantitative techniques in chemistry — and one of the most accurate.
Core Content
Gravimetric analysis is a quantitative method — it doesn't just detect a substance, it measures how much of it is present. The key idea: cause the target substance to form an insoluble precipitate with a known reagent, then collect, dry, and weigh the precipitate. Using stoichiometry (the molar ratios from the balanced equation), you can calculate the mass of the original substance.
To determine the mass of Cl⁻ ions in a solution: add excess silver nitrate (AgNO₃) solution. Cl⁻ reacts with Ag⁺ to form silver chloride (AgCl), a white precipitate that is insoluble in water.
Insert a flow diagram: Add sample solution → Add precipitating reagent → Precipitate forms → Filter (funnel + ashless paper) → Wash precipitate → Dry to constant mass → Weigh → Calculate. Use boxes with arrows, label each step briefly.
You now have five techniques. Choosing correctly is a core HSC skill. The decision always starts with the physical and chemical properties of the components you are trying to separate.
| Mixture type | Key property difference | Technique |
|---|---|---|
| Insoluble solid in liquid | Particle size | Filtration |
| Dissolved solid in solution | Solubility changes with temperature | Crystallisation |
| Liquid + non-volatile solute (large BP diff) | Boiling point | Simple distillation |
| Two miscible liquids with close BPs | Boiling point (small difference) | Fractional distillation |
| Multiple dissolved compounds (different polarities) | Differential affinity for phases | Chromatography |
| Dissolved ion — need to measure amount precisely | Forms insoluble precipitate | Gravimetric analysis |
Worked Examples
| Step 1 — Write the reactionBa²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s) Molar mass of BaSO₄ = 137.3 + 32.1 + (4 × 16.0) = 233.4 g mol⁻¹ Molar mass of SO₄²⁻ = 32.1 + (4 × 16.0) = 96.1 g mol⁻¹ |
Always write the balanced ionic equation first. The 1:1 molar ratio means 1 mol BaSO₄ came from 1 mol SO₄²⁻. This ratio is what connects the precipitate mass to the original ion. |
| Step 2 — Find moles of BaSO₄n(BaSO₄) = mass ÷ molar mass n(BaSO₄) = 0.466 ÷ 233.4 = 0.001997 mol |
Work to 4 significant figures throughout to avoid rounding errors before the final answer. |
| Step 3 — Apply molar ratioFrom the equation: n(SO₄²⁻) = n(BaSO₄) = 0.001997 mol (1:1 ratio from balanced equation) |
This 1:1 ratio is the stoichiometric bridge between what you measured (BaSO₄) and what you want to know (SO₄²⁻). In other reactions the ratio may not be 1:1 — always check the balanced equation. |
| Step 4 — Calculate mass of SO₄²⁻m(SO₄²⁻) = n × M m(SO₄²⁻) = 0.001997 × 96.1 = 0.192 g |
Round to 3 significant figures at the end (limited by the precision of the mass measurement: 0.466 g is 3 sig figs). |
| (a) Clean dry sandSand is insoluble in water. Separation basis: particle size. Technique: filtration. Pour the sample through filter paper to collect sand as residue; rinse with distilled water; dry. | The key property is insolubility. Filtration works because sand particles are large enough to be retained by filter paper, while dissolved NaCl and MgCl₂ pass through in the filtrate. |
| (b) Pure NaCl crystalsNaCl is dissolved — filtration won't work. NaCl's solubility decreases on cooling. Technique: crystallisation. First filter to remove sand (filtrate = salt water). Heat filtrate to concentrate, cool slowly to crystallise NaCl, filter crystals, dry. Note: MgCl₂ will also be present — further purification (recrystallisation) would be needed for chemically pure NaCl. | The step before crystallisation is important — you must remove the insoluble sand first, or it will be embedded in the crystals. This is an example of a sequential separation strategy. |
| (c) Separate oil from waterOil is immiscible with water (does not dissolve) and less dense (floats). The oil and water form two distinct liquid layers. Technique: separating funnel (this is an additional technique — two immiscible liquids are separated by draining the denser layer out the bottom tap while retaining the less dense layer). | This is a technique not explicitly in the main syllabus list but tests your reasoning ability. The key property is immiscibility and density difference. Accept any reasonable technique that exploits these properties. |
Activities
A chemist wants to know the exact mass of calcium ions (Ca²⁺) in a 100 mL sample of hard water. Would crystallisation or gravimetric analysis be more appropriate? Compare the two techniques and justify your choice.
A researcher needs to determine the mass of barium sulfate (BaSO₄) already present as an insoluble precipitate in a sample. Compare filtration and gravimetric analysis for this purpose.
A forensic chemist has a glass of water suspected to contain dissolved lead ions (Pb²⁺). She needs to confirm the presence of lead AND determine exactly how many milligrams of Pb²⁺ are in the sample. What technique should she use?
A student needs to separate a mixture of three amino acids from a solution. She does not need a quantitative result — she just needs to see how many components are present and whether each matches a known standard. What technique should she use?
A water treatment plant needs to remove fluoride ions (F⁻) from drinking water to below the WHO limit of 1.5 mg/L. They add calcium chloride (CaCl₂), which causes calcium fluoride (CaF₂) to precipitate. After precipitation, how do they remove the CaF₂? Describe the full procedure including how they would confirm the fluoride level after treatment.
Multiple Choice
Click to check. One attempt only.
1. What is the primary purpose of adding excess precipitating reagent in gravimetric analysis?
2. In a gravimetric analysis of chloride ions, 0.286 g of silver chloride (AgCl) precipitate is collected. What mass of Cl⁻ does this represent? [M(AgCl) = 143.3 g mol⁻¹; M(Cl) = 35.5 g mol⁻¹]
3. A student performs a gravimetric analysis and obtains a result that is higher than the actual value. Which error most likely caused this?
4. A mixture contains dissolved potassium iodide (KI) and insoluble barium sulfate (BaSO₄). A chemist wants to obtain dry, pure BaSO₄. Which technique is most appropriate as the first step?
5. Evaluate the following claim: "Gravimetric analysis is the best technique for separating and identifying all types of mixtures." Which statement best evaluates this claim?
Short Answer
6. Describe the steps involved in gravimetric analysis to determine the mass of barium ions (Ba²⁺) in a solution using sodium sulfate (Na₂SO₄) as the precipitating reagent. In your answer, explain why each step is important. 3 MARKS
7. A sample of industrial waste water is suspected to contain sulfate ions (SO₄²⁻). A chemist adds excess barium chloride solution to a 500 mL sample and collects 0.932 g of dry barium sulfate precipitate. Calculate the mass of sulfate in the 500 mL sample. Show all working. [M(BaSO₄) = 233.4 g mol⁻¹; M(SO₄²⁻) = 96.1 g mol⁻¹] 4 MARKS
8. A chemist is given a sample of sea water and asked to determine the concentration of chloride ions (Cl⁻) using gravimetric analysis. Evaluate the effectiveness of this technique for this purpose, discussing its strengths and at least two limitations. 5 MARKS
A: Crystallisation would recover NaCl crystals from the water but would not tell you the exact mass of Ca²⁺ — the dissolved calcium would remain in solution. Gravimetric analysis is more appropriate: add excess Na₂SO₄ (or Na₂CO₃) to precipitate all Ca²⁺ as CaSO₄ (or CaCO₃), filter, dry to constant mass, weigh, and use stoichiometry to calculate m(Ca²⁺). It is the only technique that provides a quantitative measurement of a specific dissolved ion.
B: Filtration could physically separate the BaSO₄ precipitate from the solution (it is already insoluble). Gravimetric analysis in this context would involve the same filtration step — but also includes washing, drying to constant mass, and weighing to obtain a quantitative result. The difference: filtration alone gives you the solid; gravimetric analysis gives you the solid AND its mass, which is required for any quantitative determination. If you only need the solid and not a precise mass, filtration alone is sufficient.
Scenario 1: Gravimetric analysis. Key property: Pb²⁺ forms an insoluble precipitate with specific reagents (e.g. add Na₂SO₄ → PbSO₄(s) precipitates). Procedure: add excess Na₂SO₄ to the water sample → PbSO₄ precipitates → filter with ashless paper → wash → dry to constant mass → weigh → use stoichiometry to calculate m(Pb²⁺). This gives both confirmation (precipitate forms) and quantification (mass calculated).
Scenario 2: Chromatography (paper or TLC). Key property: differential attraction of each amino acid to stationary/mobile phases gives different Rf values. Procedure: spot all three amino acids on the baseline along with known standards → develop with appropriate solvent → measure Rf values for each spot → compare to standard Rf values for identification. No quantitative measurement needed — separation and identification only.
Scenario 3: Remove CaF₂ by filtration — it is an insoluble precipitate. Pour the treated water through filter paper (or a membrane filter for industrial scale); CaF₂ is retained as residue; the treated water (filtrate) has reduced F⁻. To confirm fluoride level: use gravimetric analysis on a sample of the treated water — add excess CaCl₂ to precipitate any remaining F⁻ as CaF₂, dry, weigh, and calculate m(F⁻) to verify it is below 1.5 mg/L.
1. B — Excess reagent ensures complete precipitation of all target ions. Insufficient reagent leaves some ions in solution, giving an underestimate.
2. D — n(AgCl) = 0.286 ÷ 143.3 = 0.001996 mol. n(Cl⁻) = 0.001996 mol (1:1 ratio). m(Cl⁻) = 0.001996 × 35.5 = 0.0709 g.
3. C — Residual moisture adds to the measured mass, giving an overestimate. A (insufficient reagent) and B (precipitate loss) both cause underestimates.
4. A — BaSO₄ is insoluble → filtration separates it as residue. KI is dissolved → passes through in filtrate. Crystallisation, distillation, and chromatography are not suited to separating an insoluble solid from a solution in this way.
5. B — Gravimetric analysis is a quantitative measurement technique for specific ions that form insoluble precipitates. It cannot separate miscible liquids, volatile components, or immiscible liquids. D is wrong (it is still widely used); A and C are incorrect overstatements.
Q6 (3 marks): Step 1 — Add excess Na₂SO₄ to the solution: the excess ensures all Ba²⁺ ions react and precipitate as BaSO₄; without excess, some Ba²⁺ would remain dissolved and be unmeasured (1 mark). Step 2 — Filter the precipitate, wash with distilled water: filtration separates the insoluble BaSO₄ from the solution; washing removes soluble impurities that could add to the mass and cause overestimation (1 mark). Step 3 — Dry the precipitate to constant mass, then weigh: drying removes all residual water which would add falsely to the measured mass; drying to constant mass confirms no water remains; the final dry mass is used with stoichiometry to calculate m(Ba²⁺) (1 mark).
Q7 (4 marks): Reaction: Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s) [1:1 molar ratio] (1 mark). n(BaSO₄) = 0.932 ÷ 233.4 = 0.003993 mol (1 mark). n(SO₄²⁻) = 0.003993 mol (1:1 ratio from balanced equation) (1 mark). m(SO₄²⁻) = 0.003993 × 96.1 = 0.384 g (1 mark).
Q8 (5 marks): Gravimetric analysis is effective for determining Cl⁻ concentration in sea water because it can precisely quantify very small amounts of an ion, and AgCl is a highly insoluble precipitate that forms reliably and quantitatively (1 mark). Strength: it is a direct mass measurement — does not depend on colour, electronic signals, or calibration curves, making it highly accurate and reproducible (1 mark). Limitation 1: sea water contains multiple anions (SO₄²⁻, Br⁻, I⁻) that may also precipitate with AgNO₃, leading to co-precipitation and overestimation of the Cl⁻ content; additional steps to remove interfering ions are needed (1 mark). Limitation 2: the procedure is time-consuming — drying to constant mass can take hours to days; modern methods like ion chromatography or potentiometric titration are faster for routine analysis (1 mark). Overall: gravimetric analysis is highly accurate for Cl⁻ in sea water if interfering ions are controlled, but is not ideal for high-throughput or field testing due to time requirements (1 mark).
Tick when you've finished all activities and checked your answers.