Gravimetric Analysis Virtual Lab: Calcium in Hard Water

This gravimetric analysis virtual lab follows the hard water investigation of AP Chemistry and the gravimetric work of IB and VCE Chemistry. Students measure 500 cm³ of water, precipitate the calcium as calcium carbonate with sodium carbonate solution, test the clear liquid until precipitation is complete, then filter, wash, and heat, cool and weigh until two readings agree. From the mass of the precipitate they calculate the hardness in mg CaCO₃ per dm³ and the calcium content in mg Ca²⁺ per dm³. A standard solution checks the method, and skipping a step shows its effect straight away. Every number below was read from the simulation.

Curriculum links

• AP Chemistry Unit 1, topic 1.4 (composition of mixtures, SPQ-2.B) and the AP gravimetric analysis investigation on hard water.
• IB Chemistry, Tool 1 (experimental techniques): measuring mass, filtration and heating to constant mass, with the mole calculations of Structure 1.4.
• VCE Chemistry Unit 2, Area of Study 2: measuring substances in water, including gravimetric analysis.

Simulic is not affiliated with or endorsed by the College Board, the IB or the VCAA.

Before the lab (5 min)

Ask: "A student forgets to wash the calcium carbonate precipitate before drying it. Will the hardness they calculate be too high, too low or correct?" On a class link this is question 1; the simulation unlocks after students answer.

Method in the simulation

1. Choose Hard water: Ca²⁺ as CaCO₃, then Standard solution, and set Balance to 0.0001 g. Keep the Water sample slider at 500 cm³.
2. Press Measure the water, then Add 10 cm³ Na₂CO₃. Press Test clear liquid; add more Na₂CO₃ until no new cloudiness appears.
3. Press Filter, then Wash twice.
4. Press Heat, cool, weigh until the message says the last two readings agree, then Record result.
5. Repeat without washing. Then choose Water sample to analyse the unknown tap water.

Run	Sample	Na₂CO₃ added (cm³)	Washes	Heatings	Mass of CaCO₃ (g)	Hardness (mg CaCO₃/dm³)	Ca²⁺ (mg/dm³)
1	Standard		2
2	Standard		0
3	Water sample		2

Expected results

The standard contains 400 mg CaCO₃ per dm³, so 500 cm³ holds 0.2000 g. Balance readings carry a tiny random error, so results agree closely.

• Standard, one portion of Na₂CO₃, washed twice: 0.2009 g CaCO₃, 402 mg/dm³ and 161 mg Ca²⁺/dm³. Constant mass came after 5 heatings. With two portions: 0.2016 g, 403 mg/dm³.
• Washed once: 0.2037 g, 407 mg/dm³. Not washed: 0.2147 g, 429 mg/dm³, flagged "not washed". The leftover soluble salts add to the mass.
• Balance 0.01 g: 0.20 g and 400 mg/dm³, but the uncertainty is ±0.02 g, 10 % of the mass.
• Water sample: a different unknown for every page load or New unknown. One run gave 185 mg/dm³ against a true value of 180 mg/dm³.

Questions for students

1. (Prediction, asked again after the lab) If the precipitate is not washed, is the calculated hardness too high, too low or correct?
2. A class analyses the same water using 250, 500 and 1000 cm³ samples. Which is the dependent variable?
3. What mass of calcium carbonate do you get from 500 cm³ of the standard solution?
4. Calculate the calcium content of the standard in mg Ca²⁺ per dm³.
5. Explain why you heat, cool and weigh until two readings agree.

Answers for teachers: (1) Too high: 429 instead of about 402 mg/dm³. (2) The mass of dry calcium carbonate. (3) Accept 0.198–0.207 g (about 0.201 g). (4) Mass ÷ 100.09 × 40.08 × 1000 mg ÷ 0.500 dm³; accept 158–166 mg/dm³. (5) Each heating drives off more water. Two equal readings show that all the water has gone; stopping early leaves water in the solid, so the mass and the result are too high.

Common misconceptions

• "More reagent always gives a better result." Enough is needed for complete precipitation, but a large excess leaves more soluble salt to wash out.
• "Washing loses precipitate." Calcium carbonate is insoluble; washing removes only the soluble salts.
• "One heating is enough." The first weighing still includes most of the water held by the filter cake.
• "The balance's last digit makes the result exact." A 0.01 g balance gives 0.20 g for a 0.2009 g precipitate.

Extension

• Sulfate as BaSO₄: with Pure substance, 1 g of ammonium sulfate gave 72.8 % sulfate (true 72.7 %); unwashed, 74.0 %. Three portions of BaCl₂ left precipitation incomplete: 57.8 %.
• Hydrated barium chloride: heat to constant mass and find x in BaCl₂·xH₂O.

FAQ

Can I pin the settings for my class?

Yes. On the Share page, pin Experiment to "Calcium in hard water, precipitated as CaCO₃", Sample to "Pure substance / standard solution", Balance resolution to "0.0001 g (analytical balance)" and Volume of water sample to 500 cm³ in the link's starting values.

Does every student get the same water sample?

No. The unknown water sample is chosen at random each time the page loads, so each student finds a different hardness and compares it with the true value in the table. That is why the marked questions use the standard solution.

Why does the table show the true value?

So students can work out their percentage error and link it to the steps they skipped.

Related simulations and guides

Identifying ions in salts – flame tests and precipitate tests Limiting Reactant and Percentage Yield – Stoichiometry with Particles

For heating to constant mass with magnesium, see the empirical formula virtual lab. For precipitate tests, see the identifying ions virtual lab.