# Stoichiometry and a precipitation reaction

1. Review the following reaction, where sodium carbonate and calcium chloride dihydrate react in an aqueous solution to create calcium carbonate (solid precipitate formed in the reaction), a salt (sodium chloride), and water.

Na2CO3(aq) + CaCl2⋅2H2O → CaCO3(s) + 2NaCl(aq) + 2H2O(aq) Na2CO3(aq) + CaCl2⋅2H2O → CaCO3(s) + 2NaCl(aq) + 2H2O(aq)

1. Put on your safety gloves and goggles.
2. Use the graduated cylinder to measure 25 ml of distilled water. Add 25 ml of distilled water to each of the two 100 ml glass beakers.
3. Turn on the digital scale, place the plastic weigh boat on the scale and tare the scale so that it reads 0.00 g. See Figure 3.
4. Use the metal spatula to measure 1.50 grams of the CaCl2•2H2O.
5. Carefully add the CaCl2•2H2O to one of the beakers containing 25 mL of distilled water. Submerge the weigh boat into the beaker to transfer all of the CaCl2•2H2O if any residue remains on the weigh boat. Swirl the beaker until the CaCl2•2H2O is fully dissolved into the water.
6. Rinse the weigh boat and metal spatula with distilled water and fully dry both with paper towels.
7. Use the information and examples provided in the Exploration to calculate how many moles of CaCl2•2H2O are present in 1.50 g of CaCl2•2H2O and then calculate how many moles of pure CaCl2 are present in the 1.50 g of CaCl2•2H2O. Record the answers in Data Table 1.
8. Use the information and examples provided in the Exploration and the values recorded in Data Table 1 from Step 8 to determine how many moles of Na2CO3 are necessary to reach stoichiometric quantities. From that calculation, determine how many grams of Na2CO3 are necessary to reach stoichiometric quantities. Record both values in Data Table 1.
9. Turn on the digital scale, place the plastic weigh boat on the scale and tare the scale so that it reads 0.00 g.
10. Use the metal spatula to measure the calculated amount of Na2CO3, and carefully add it to the 25 mL of distilled water in the second 100 mL glass beaker. Submerge the weigh boat into the beaker to transfer all of the Na2CO3 if any residue remains on the weigh boat.
11. Use the stir rod to stir the Na2CO3  and break apart any clumps until it is fully dissolved into the water.
12. Pour the Na2CO3 solution from the 100 mL glass beaker into the beaker containing the CaCl2•2H2O solution. Rinse the beaker containing Na2CO3 with 2-3 mL of distilled water and transfer the rinse to the beaker containing the CaCl2•2H2O. Swirl the beaker to fully mix the two solutions and the precipitate of calcium carbonate will form instantly.
13. Use the information and examples provided in the Exploration to determine the maximum (theoretical) amount of CaCO3, in grams, that can be produced from the precipitation reaction. Record this value in Data Table 1.
14. Wash the now empty 100 mL glass beaker (that contained the Na2CO3 solution) with soap and water. Rinse the beaker with distilled water and thoroughly dry with paper towel.
15. Fold the round filter paper into a cone shape, as shown in Figure 3.
1. Place the folded filter paper onto the tared scale and record the mass of the filter paper in Data Table 1.
2. Place the folded filter paper into the funnel and dampen it with a small amount of distilled water. Swirl the contents of the beaker to dislodge any precipitate from the sides and while holding the filter paper open, slowly pour the contents of the beaker into the filter-paper lined funnel.

Note: Be careful not to overfill the funnel. It may be necessary to gently swirl the funnel to keep the precipitate from clogging the paper, but be careful not to touch the filter paper so that it does not tear.

1. Add 2-3 mL of distilled water to the beaker and swirl the water around the sides of the beaker to collect any precipitate stuck to the sides of the beaker. Pour into the filter-paper lined funnel.
2. Allow all of the liquid to drain from the funnel into the beaker. This may take 10-15 minutes.

Note: Lift the funnel periodically during this time to facilitate the water draining into the beaker. Again, do not touch the filter paper so that it does not tear.

1. After all liquid has drained from the funnel, carefully remove the filter paper from the funnel and place it on paper towels in a warm location, such as a window that receives a lot of sunlight, where it will not be disturbed. See Figure 4.
1. Allow the filter paper to completely dry, which will require an overnight drying period.

Note: Ensure the filter paper is placed in a location that does not have flowing air, such as a fan or air vent.

1. When the filter paper with precipitate is completely dry, tare the scale and place the paper onto the scale to obtain the mass. Record the mass of the filter paper and precipitate in Data Table 1.
2. Calculate the actual mass of the precipitate and record in Data Table 1.
3. Calculate the percent yield of the precipitate and record in Data Table 1.