Answer to Question #74515 in General Chemistry for Arianna Cole

Question

Outline the laboratory preparation of a pure, dry sample of CaCO3, include in your response a balanced chemical equation.

Answer 1

While calcium carbonate (CaCO3) is sparingly soluble in water, both Na2CO3 and CaCl2 have much higher solubility. By reacting the soluble salts, we generate a precipitate that can be isolated from any remaining soluble salts by sequential centrifugation and filtration.

Na2CO3(aq) + CaCl2(aq) → CaCO3(s) + 2NaCl(aq)

The identity of the salt can be corroborated by qualitative chemical tests.

Method:

1. Setup one burette filled with the 1.0 M CaCl2 (aq) solution.

2. Pipette 5.0 cm3 1.0 M Na2CO3 (aq) into the test tube.

3. Deliver 6.0 cm3 1.0 M CaCl2 (aq) to the same test tube using the burette.

4. To separate the solid, centrifuge the solution for 5 minutes.

5. While the centrifuge is going on, setup a Buchner funnel.

6. Fit the Buchner funnel with a filter paper of the appropriate size, and weigh the filter paper on a balance. Then replace into the funnel, moistening with water to ensure adquate contact. Test using the water aspirator.

7. Once the centrifugation is complete, decant the solution through the funnel while the setup is under reduced pressure. Transfer the remaining solid in the test tube using the minimal amount of water. Note the appearance of the filtrate (solution on the bottom).

8. Allow the filter paper to dry on the aspirator for 2 minutes, then transfer the damp filter paper to a watch-glass.

9. The watch glass, together with the filter paper, will be dried in the oven for you.

10. Weigh and record the mass of the sample and filter paper.

11. Perform the identification test on your sample.