Chemistry • Year 11 • Module 2 • Lesson 9
Gravimetric Analysis
Lock in the vocabulary, five-step procedure, and core calculation pathway before tackling harder questions.
1. Term–definition match
The definitions below are shuffled. In the right-hand column write the matching term from this list: gravimetric analysis, precipitating agent, filtration, ignition, mole ratio, analyte, coprecipitation, desiccator, quantitative filtration, excess reagent. 10 marks (1 each)
| # | Definition | Matching term |
|---|---|---|
| 1.1 | A quantitative technique that determines the amount of a substance by converting it to a pure, insoluble solid and measuring the mass of that solid. | |
| 1.2 | The substance whose amount or concentration is being measured in a chemical analysis. | |
| 1.3 | A reagent added in slight excess to ensure complete precipitation of the target ion from solution. | |
| 1.4 | The separation of precipitate from solution using ashless filter paper so the mass of precipitate is accurately measured. | |
| 1.5 | Heating a precipitate in a crucible to drive off residual water and convert it to a stable stoichiometric form. | |
| 1.6 | The stoichiometric relationship (from a balanced equation) between moles of precipitate and moles of analyte. | |
| 1.7 | Adding more of the precipitating reagent than the stoichiometric amount, to drive the reaction to completion. | |
| 1.8 | A sealed container holding a drying agent (e.g. silica gel) used to cool precipitates without reabsorption of atmospheric moisture. | |
| 1.9 | An error where unwanted impurities are carried down with the precipitate, increasing its mass above the true value. | |
| 1.10 | The amount that remains of the precipitating reagent after all target ions have reacted; it stays dissolved and is washed away. |
2. True or false — with correction
Circle T or F for each statement. If the statement is false, write the corrected version on the line below it. 12 marks (1 T/F + 1 correction each)
2.1 In gravimetric analysis, you divide the mass of the precipitate by the molar mass of the analyte to find the moles of precipitate. T / F
2.2 BaSO4 and AgCl are both extremely insoluble solids, making them suitable precipitates for gravimetric analysis. T / F
2.3 The precipitate should be weighed while it is still warm from the oven so that atmospheric moisture does not condense on it during cooling. T / F
2.4 Adding excess precipitating reagent drives the reaction to completion, ensuring all the analyte is converted to precipitate. T / F
2.5 If the filter paper mass is not subtracted from the total dried mass, the calculated precipitate mass will be too low. T / F
2.6 For the reaction Ba2+(aq) + SO42−(aq) → BaSO4(s), the mole ratio of BaSO4 to SO42− is 1 : 1. T / F
3. Fill-in-the-blank paragraph
Use the word bank to complete the passage. Each word is used once. 8 marks (1 per blank)
Word bank:
balance · complete · concentration · desiccator · excess · insoluble · mole ratio · precipitate
Gravimetric analysis determines the amount of an analyte by converting it into a pure, ___________ solid called a ___________. A slight ___________ of the precipitating reagent is added to drive the reaction to ___________, ensuring all of the analyte reacts. After filtering and washing, the precipitate is dried and cooled in a ___________ before being weighed on an analytical ___________. The ___________ from the balanced equation connects moles of precipitate to moles of analyte, allowing the ___________ of the analyte in the original sample to be calculated.
4. Function recall
Answer each question in 1–2 sentences using precise terms from the lesson. 8 marks (2 each)
4.1 Why must the precipitate be extremely insoluble for gravimetric analysis to give an accurate result?
4.2 What is the function of washing the precipitate with distilled water during filtration?
4.3 Why is a desiccator used rather than simply allowing the precipitate to cool on the bench after oven drying?
4.4 State the four-step calculation chain that links mass of precipitate to concentration of analyte.
5. Sequence the gravimetric procedure
The five steps of gravimetric analysis are listed below in scrambled order. Write the correct step number (1–5) in the “Order” column, then complete the “Reason (one sentence)” column. 10 marks (1 order + 1 reason each)
| Order | Scrambled step description | Reason (one sentence) |
|---|---|---|
| Weigh the cooled precipitate precisely and subtract the filter paper / crucible mass to get the net precipitate mass. | ||
| Add a slight excess of the precipitating reagent to the sample solution and stir until all the analyte has reacted. | ||
| Dissolve the solid sample in an appropriate solvent to bring the analyte into aqueous solution. | ||
| Filter the mixture through pre-weighed filter paper and wash the precipitate with a small volume of distilled water. | ||
| Dry the filter paper and precipitate in an oven at ~120 °C, then cool in a desiccator. |
Q1 — Term-definition match
1.1 gravimetric analysis • 1.2 analyte • 1.3 precipitating agent • 1.4 quantitative filtration • 1.5 ignition • 1.6 mole ratio • 1.7 excess reagent • 1.8 desiccator • 1.9 coprecipitation • 1.10 excess reagent (accept “the dissolved excess of the precipitating agent”).
Q2 — True / false with correction
2.1 False. You divide by the molar mass of the precipitate (not the analyte) in Step 1 to obtain moles of precipitate. The analyte MM is only used later if the question asks for the mass of analyte.
2.2 True. Both BaSO4 and AgCl have extremely low solubility products, ensuring nearly complete precipitation.
2.3 False. The precipitate must be cooled in a desiccator before weighing. Weighing a warm object causes convection currents above the balance pan, reducing the apparent mass reading (buoyancy effect), and the hot surface can absorb moisture from the air.
2.4 True.
2.5 False. If the filter paper mass is not subtracted, the calculated precipitate mass will be too high (the filter paper adds mass to the reading).
2.6 True. The balanced ionic equation Ba2+ + SO42− → BaSO4 has a 1 : 1 mole ratio.
Q3 — Cloze paragraph
In order: insoluble / precipitate / excess / complete / desiccator / balance / mole ratio / concentration.
Q4.1 — Why the precipitate must be insoluble
If the precipitate has any appreciable solubility, some of the analyte remains dissolved in solution rather than being collected on the filter. This means the mass weighed underestimates the true amount of analyte, giving a result that is too low. The more insoluble the precipitate, the more complete the reaction and the more accurate the result.
Q4.2 — Function of washing during filtration
Washing removes soluble impurities (such as the excess precipitating reagent and other dissolved ions) from the surface of the precipitate without dissolving the precipitate itself, since the wash solvent (distilled water) contains no ions that would disturb the equilibrium of the highly insoluble solid.
Q4.3 — Why a desiccator is used
After oven drying, the hot precipitate would absorb water from atmospheric moisture if cooled on the open bench. The desiccator contains a drying agent (e.g. silica gel) that removes moisture from the air inside the sealed container, so the precipitate cools without picking up any additional mass from water vapour.
Q4.4 — Four-step calculation chain
(1) m(precipitate) ÷ MM(precipitate) = n(precipitate). (2) n(precipitate) × mole ratio = n(analyte). (3) n(analyte) × MM(analyte) = m(analyte). (4) n(analyte) ÷ V(solution) = c(analyte).
Q5 — Correct order and reasons
Step 1: Dissolve the sample (third row in table). Reason: The analyte must be in aqueous solution for the precipitating reagent to react with it uniformly.
Step 2: Add excess precipitating reagent (second row). Reason: Drives the precipitation reaction to completion, ensuring all analyte is converted to precipitate.
Step 3: Filter the mixture (fourth row). Reason: Separates the insoluble precipitate from the solution; washing removes soluble impurities.
Step 4: Dry the precipitate (fifth row). Reason: Removes all residual water, which would add mass and cause the analyte amount to be overestimated.
Step 5: Weigh and calculate (first row). Reason: The net precipitate mass is the data from which all calculations are made; precision here determines the accuracy of the final result.