Year 12 Chemistry Module 8 · IQ1 ⏱ ~35 min Lesson 2 of 19

Gravimetric Analysis

A water authority receives an industrial wastewater sample and needs to know whether sulfate levels are too high for safe discharge. No fancy colour change, no spectrometer screen, just chemistry: dissolve, precipitate, filter, dry, weigh, and let the mass reveal what was present.

Tripod with gauze
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Think First

Case Entry Before Method

An environmental chemist adds barium ions to a wastewater sample and a fine white solid appears. After filtering and drying that solid, the chemist uses only its mass to judge the sulfate content of the original water.

  • Why might weighing a solid precipitate be enough to determine how much sulfate ion was in the sample?
  • What parts of the process could make the final measured mass too high or too low?

📖 Know

  • The sequence dissolve → precipitate → filter → dry → weigh
  • How gravimetric analysis determines the amount of analyte from precipitate mass
  • Which precipitating agents are suitable for common ions

💡 Understand

  • Why an insoluble, pure precipitate is essential for reliable gravimetric analysis
  • How stoichiometry links precipitate mass to original analyte amount
  • How incomplete precipitation, co-precipitation, incomplete drying and filtration losses change results

✅ Can Do

  • Calculate analyte mass or percentage composition from precipitate data
  • Select a suitable precipitating agent for Cl-, SO42- and CO32-
  • Interpret gravimetric results and judge whether a data set is reliable

📚 Core Content

Key Terms — scan these before reading
know whether sulfate levelstoo high for safe discharge
Which precipitating agentssuitable for common ions
pure precipitateessential for reliable gravimetric analysis
the correct orderdissolve, precipitate, filter, dry, then weigh
The precipitatefiltered and dried
Gravimetric analysischemistry reduced to its most disciplined form: if you can isolate a pure precipitate of known composition, its mass bec

Choose how you work — type your answers below or write in your book.

1

What Gravimetric Analysis Measures

Use the mass of a known precipitate to infer the mass of an unknown analyte

Gravimetric analysis is chemistry reduced to its most disciplined form: if you can isolate a pure precipitate of known composition, its mass becomes a direct clue to the original sample.

In gravimetric analysis, the ion or compound of interest is converted into an insoluble precipitate with a known chemical formula. Once that precipitate has been collected and dried, its mass can be used to calculate the amount of analyte originally present.

For sulfate analysis, adding Ba2+(aq) produces barium sulfate, BaSO4(s), a very insoluble white precipitate:

Core Gravimetric Logic

Ba2+(aq) + SO42-(aq) → BaSO4(s) One mole of sulfate ion produces one mole of barium sulfate precipitate.
n = m / M Moles of precipitate = mass divided by molar mass
% composition = (mass of analyte / mass of sample) × 100 Once analyte mass is found, percentage composition follows directly.
Tripod with gauze

Tripod and gauze — for heating crucibles to constant mass

Bunsen burner

Bunsen burner — provides heat for drying precipitates

Misconceptions to Fix

✗

Wrong: Heavy metals are dangerous because they are radioactive.

✗

Right: Heavy metals are toxic because they bioaccumulate and interfere with enzyme function, not because of radioactivity.

HSC languageWhen describing gravimetric analysis, refer to conversion of the analyte into an insoluble precipitate of known composition, followed by isolation, drying and weighing so stoichiometric calculations can determine the original amount present.
1. Dissolve 2. Precipitate 3. Filter 4. Dry 5. Weigh 60°C 0.500 g get analyte into solution form insoluble solid separate precipitate remove trapped water mass gives moles

Gravimetric analysis succeeds only if the precipitate is fully formed, completely collected, thoroughly dried, and then weighed accurately. Each step contributes directly to the final chemical inference.

2

The Practical Process

Dissolve → precipitate → filter → dry → weigh

A gravimetric result is only as good as the technique used to isolate the solid. The chemistry may be simple, but the method is unforgiving.

  1. Dissolve: Ensure the analyte is fully in solution so it can react completely.
  2. Precipitate: Add an appropriate reagent to form a low-solubility precipitate.
  3. Filter: Separate the solid from the liquid without losing precipitate.
  4. Dry: Remove water so the mass measured is the mass of the precipitate, not liquid trapped with it.
  5. Weigh: Measure the dry precipitate accurately and use stoichiometry to work backwards.

If any step is incomplete, the final mass no longer represents the true amount of precipitate formed. Gravimetric analysis therefore depends on both chemical selectivity and careful laboratory technique.

Industry anchorIn wastewater testing, gravimetric analysis is especially useful when the target ion forms a stable insoluble salt. For sulfate discharge monitoring, a well-dried BaSO4(s) precipitate can provide an inexpensive and defensible measure of contamination.
3

Choosing the Right Precipitating Agent

Solubility rules decide whether the method will work

A gravimetric method succeeds only when the reagent produces a precipitate that is both sufficiently insoluble and chemically specific.

Cl-

Precipitating reagent: AgNO3(aq)
Precipitate formed: AgCl(s)
Why it works: Silver chloride is insoluble and forms a distinct solid

SO42-

Precipitating reagent: BaCl2(aq) or Ba(NO3)2(aq)
Precipitate formed: BaSO4(s)
Why it works: Barium sulfate has very low solubility

CO32-

Precipitating reagent: CaCl2(aq)
Precipitate formed: CaCO3(s)
Why it works: Calcium carbonate precipitates from solution

A precipitating reagent must not simply “make a solid”. It must form a precipitate with known composition, low solubility, and minimal side reactions with other ions in solution.

Common error“Any reagent that makes cloudiness is fine.” Cloudiness alone is not enough. The precipitate must be identifiable, sufficiently insoluble, and suitable for accurate stoichiometric conversion to the analyte.
4

Calculations from Precipitate Mass

Mass of precipitate → moles of precipitate → moles of analyte → mass or percentage

The reliable way to solve gravimetric questions is to convert through moles. Do not jump straight from precipitate mass to percentage by intuition.

  1. Measure the mass of dry precipitate formed.
  2. Calculate moles of precipitate using n = m / M.
  3. Use the balanced equation to find moles of analyte.
  4. Convert moles of analyte into mass using m = nM.
  5. If needed, calculate percentage composition from the original sample mass.

For BaSO4(s), the mole ratio to SO42- is 1:1. That makes sulfate gravimetric analysis especially clean: one mole of precipitate corresponds to one mole of sulfate ion in the original sample.

5

Sources of Error in Gravimetric Analysis

Why the final mass can be too low or too high

A gravimetric result looks objective because it ends with a balance reading, but that reading can still be wrong for several chemical reasons.

Error source What happens Effect on measured mass Effect on result
Incomplete precipitation Not all analyte forms the solid Too low Analyte amount underestimated
Co-precipitation Other ions or impurities become trapped in the precipitate Too high Analyte amount overestimated
Incomplete drying Water remains in or on the precipitate Too high Analyte amount overestimated
Loss on filtration Some precipitate passes through or is left behind Too low Analyte amount underestimated
Must knowIn HSC responses, always link the error to its direction. Saying “incomplete drying is an error” is not enough; you should say it causes the measured mass to be too high, so the analyte amount is overestimated.
Final measured precipitate mass Too low Too high Incomplete precipitation Not all analyte becomes solid, so analyte is underestimated. Filtration loss Some precipitate is lost, so moles appear smaller. Co-precipitation Impurities add extra mass, so analyte is overestimated. Incomplete drying Water remains mass too high

The key HSC move is to connect each procedural error to its direction: low measured mass causes underestimation, while extra mass from water or impurities causes overestimation.

📊 Data Interpretation

D

Wastewater Sulfate Data Set

Interpreting mass data from an industrial discharge sample

A 0.500 g dried wastewater residue was dissolved and treated with excess BaCl2(aq). The precipitated BaSO4(s) was filtered, dried and weighed in three trials.

Trial Mass of dry sample / g Mass of BaSO4 precipitate / g Observation
1 0.500 0.348 White precipitate, dried to constant mass
2 0.500 0.351 White precipitate, dried to constant mass
3 0.500 0.392 Sample removed from oven early; still slightly damp

Trials 1 and 2 are consistent and credible. Trial 3 is likely too high because incomplete drying leaves extra water in the precipitate, inflating the measured mass. A chemist would not average all three results blindly.

InterpretModule 8 questions often ask you to do more than compute. Here, the strongest answer identifies the valid trials, explains why one trial is unreliable, and links the procedural note directly to the direction of error.

✏️ Worked Example

Worked Example

Finding Sulfate Percentage by Gravimetric Analysis

1

Given: A 0.500 g sample produces 0.350 g of BaSO4(s).

Ba2+(aq) + SO42-(aq) → BaSO4(s)

Molar mass of BaSO4 = 233.39 g mol-1. Molar mass of SO42- = 96.06 g mol-1.

2

Find: Mass and percentage of sulfate ion in the original sample.

3

Method: Calculate moles of BaSO4.

n(BaSO4) = m / M = 0.350 / 233.39 = 0.00150 mol

The equation ratio is 1:1, so:

n(SO42-) = 0.00150 mol

Convert sulfate moles to sulfate mass.

m(SO42-) = nM = 0.00150 × 96.06 = 0.144 g

Now find percentage composition of sulfate in the 0.500 g sample.

% sulfate = (0.144 / 0.500) × 100 = 28.8%

Answer: The sample contains 0.144 g of sulfate ion, which is 28.8% by mass.

Try It Now

Quick Gravimetric Calculation

A 0.800 g fertiliser sample produces 0.287 g of AgCl(s) after treatment with AgNO3(aq). Calculate the mass of chloride ion in the sample and then its percentage by mass.

📘 Copy Into Your Books

▼

Definition

  • Gravimetric analysis determines the amount of an analyte from the mass of a precipitate of known composition.
  • The precipitate must be insoluble, pure and dry.

Method

  • Dissolve the sample.
  • Precipitate the analyte.
  • Filter, dry and weigh the precipitate.

Calculation Path

  • Mass of precipitate → moles of precipitate using n = m/M.
  • Use stoichiometry to find analyte moles.
  • Convert to analyte mass or percentage composition.

Key Errors

  • Incomplete precipitation and filtration loss make mass too low.
  • Co-precipitation and incomplete drying make mass too high.

🧠 Activities

Calculate + Interpret — Activity 1

Calculating Sulfate from Wastewater Data

Use the wastewater table above. Select the valid trials, justify that choice, then calculate the sulfate percentage in the dried residue.

1 Which trial should be excluded, and what specific procedural issue makes it unreliable?

2 Calculate the average valid mass of BaSO4(s).

3 Calculate the sulfate percentage in the dried residue using the average valid mass.

Calculate + Interpret — Activity 2

Choosing Reagents and Diagnosing Errors

For each scenario, choose the best precipitating reagent or identify the most likely gravimetric error and explain the direction of its effect.

1 A chemist wants to determine chloride concentration in river water by gravimetric analysis. Which reagent should be added, and what precipitate forms?

2 A precipitate is weighed before it is fully dry. Explain the effect on the measured mass and the final analyte calculation.

3 During filtration, some of the solid passes through torn filter paper. Explain the effect on the result.

4 Why is Ba2+(aq) preferred over Na+(aq) for sulfate gravimetric analysis?

Interactive
Interactive: Spectra Matcher Interactive
Multiple Choice
?

Test Your Understanding

Think like an analyst, not just a calculator
UnderstandBand 3

1. Which sequence best describes the core gravimetric analysis process?

A
Dissolve → weigh → precipitate → dry → filter
B
Dissolve → precipitate → filter → dry → weigh
C
Filter → precipitate → dissolve → dry → weigh
D
Precipitate → dissolve → filter → weigh → dry
B
Dissolve → precipitate → filter → dry → weigh
C
Filter → precipitate → dissolve → dry → weigh
D
Precipitate → dissolve → filter → weigh → dry
ApplyBand 4

2. Which reagent is most suitable for precipitating sulfate ion in a gravimetric analysis?

A
NaCl(aq), because sodium salts are always easy to dissolve
B
HCl(aq), because acids remove sulfate from solution
C
NaNO3(aq), because nitrate does not interfere with sulfate
D
BaCl2(aq), because it forms insoluble BaSO4(s)
B
HCl(aq), because acids remove sulfate from solution
ApplyBand 4

3. A precipitate is not dried fully before weighing. What is the most likely effect?

A
The measured mass is too high, so the analyte amount is overestimated
B
The measured mass is too low, so the analyte amount is underestimated
C
There is no effect because water is not included in balance readings
D
The measured mass is unchanged, but the precipitate becomes more soluble
ApplyBand 4

A precipitate is not dried fully before weighing. Identify the most likely effect?

A
The measured mass is too high, so the analyte amount is overestimated
B
The measured mass is too low, so the analyte amount is underestimated
C
There is no effect because water is not included in balance readings
D
The measured mass is unchanged, but the precipitate becomes more soluble
AnalyseBand 5

4. Which statement best describes co-precipitation?

A
Not enough reagent is added, so some analyte remains dissolved
B
The precipitate is lost during transfer, lowering the final mass
C
Impurities become trapped with the precipitate, increasing the measured mass
D
The analyte reacts with water instead of the precipitating reagent
B
The precipitate is lost during transfer, lowering the final mass
C
Impurities become trapped with the precipitate, increasing the measured mass
D
The analyte reacts with water instead of the precipitating reagent
AnalyseBand 5

5. A student calculates sulfate content from all three wastewater trials, including the damp precipitate result. Why is this poor analytical practice?

A
Because gravimetric analysis should never use repeat trials
B
Because the damp trial is a known procedural outlier that makes the precipitate mass artificially high
C
Because the largest mass is always the least reliable result
D
Because sulfate cannot be determined from barium sulfate mass
B
Because the damp trial is a known procedural outlier that makes the precipitate mass artificially high
C
Because the largest mass is always the least reliable result
D
Because sulfate cannot be determined from barium sulfate mass
Short Answer
SA

Short Answer Practice

Use chemistry plus judgement
ApplyBand 4

1. Describe how a chemist would determine the percentage composition of sulfate in a wastewater sample using gravimetric analysis. In your answer, refer to precipitation, isolation of the solid, and calculation steps. 4 marks

AnalyseBand 5

2. Explain how incomplete precipitation and loss of precipitate on filtration would each affect the final calculated analyte content. 4 marks

EvaluateBand 5-6

3. Evaluate the suitability of gravimetric analysis for monitoring sulfate concentration in industrial wastewater. In your answer, refer to one strength of the method, one limitation or error risk, and whether the method provides enough evidence for environmental decision-making. 5 marks

Revisit Your Thinking

Go back to your opening case-entry response and tighten it into full analytical chemistry language.

✅ Comprehensive Answers

▼

Try It Now

Ag+(aq) + Cl-(aq) → AgCl(s), so the mole ratio is 1:1.

Molar mass of AgCl = 143.32 g mol-1.

Step 1: n(AgCl) = 0.287 / 143.32 = 0.00200 mol.

Step 2: n(Cl-) = 0.00200 mol.

Step 3: m(Cl-) = 0.00200 × 35.45 = 0.0710 g.

Step 4: % chloride = (0.0710 / 0.800) × 100 = 8.88%.

Activity 1

1. Exclude Trial 3 because the note states the precipitate was still slightly damp. Incomplete drying makes the measured mass too high, so sulfate content would be overestimated.

2. Average valid mass = (0.348 + 0.351) / 2 = 0.3495 g.

3. n(BaSO4) = 0.3495 / 233.39 = 0.00150 mol. Therefore n(SO42-) = 0.00150 mol. m(SO42-) = 0.00150 × 96.06 = 0.144 g. % sulfate = (0.144 / 0.500) × 100 = 28.8%.

Activity 2

1. Use AgNO3(aq). It forms AgCl(s), an insoluble silver chloride precipitate.

2. If the precipitate is not fully dry, extra water is included in the balance reading. The measured mass is too high, so the analyte amount is overestimated.

3. Losing precipitate during filtration makes the final mass too low, so the analyte amount is underestimated.

4. Ba2+(aq) is preferred because it forms insoluble BaSO4(s). Sodium sulfate remains soluble, so Na+(aq) would not produce a useful gravimetric precipitate.

Multiple Choice

1. B — the correct order is dissolve, precipitate, filter, dry, then weigh.

2. D — BaCl2(aq) forms insoluble BaSO4(s).

3. A — incomplete drying adds water mass and makes the result too high.

4. C — co-precipitation means impurities are trapped with the precipitate, increasing measured mass.

5. B — the damp trial is a known outlier with a specific chemical reason for being too high.

Short Answer Model Answers

Q1 (4 marks): The wastewater sample is dissolved so the sulfate ions are in solution. A solution containing Ba2+(aq), such as BaCl2(aq), is added to form BaSO4(s), an insoluble white precipitate. The precipitate is then filtered, dried thoroughly, and weighed. Its mass is converted to moles using n = m/M, and because the mole ratio between BaSO4 and SO42- is 1:1, the moles and mass of sulfate in the original sample can be calculated. Percentage composition is then found using (mass of sulfate / mass of original sample) × 100.

Q2 (4 marks): Incomplete precipitation means some analyte remains dissolved instead of forming the solid precipitate. This makes the measured precipitate mass too low, so the analyte content is underestimated. Loss of precipitate on filtration also reduces the final mass because some solid is physically lost before weighing. This again causes the analyte content to be underestimated. Although the causes are different, both errors lower the measured precipitate mass and therefore the calculated result.

Q3 (5 marks): Gravimetric analysis is suitable for monitoring sulfate in industrial wastewater because sulfate forms a very insoluble precipitate, BaSO4(s), allowing a direct stoichiometric link between precipitate mass and sulfate amount. A major strength is that the method is simple, inexpensive and based on measurable mass rather than subjective colour intensity. However, it is vulnerable to procedural errors such as incomplete drying or co-precipitation, both of which can distort the result. Overall, gravimetric analysis provides strong evidence for environmental monitoring when the precipitate is pure and dried to constant mass, especially if repeat trials are consistent and any outliers are justified scientifically.

🏎️
Speed Race

Race Through Gravimetric Analysis!

Answer questions about precipitation, filtering and mass calculations. Pool: lessons 1–2.

Mark lesson as complete

Tick when you've finished the activities and checked your answers.

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