Chemistry Core Lab “Calculating the Mass of a Precipitate”

This is one of three experiments that I use to determine the mass of a precipitate. This takes the students 2 class periods over the course of 3 days to accomplish. I do not have access to vacuum filtration and dryers so the final results from this lab may take a week as the precipitate needs to be completely dry. I demo the procedure during one class period because they have not been exposed to filtration techniques. I then give each lab group the mass of one reactant to measure out and then they have to calculate the exact mass of the other before I will allow them into the lab. I like to use sodium carbonate as one reactant and calcium chloride as the other. Both are very soluble in water and the precipitate formed can be easily filtered. By this time in the course, the students are writing molecular equations, ionic equations and net ionic equations. They also understand what spectator ions are. I make them identify the precipitate formed and analyze the mass expected for the ppt.

The students make their own solutions. Not all groups will have the same reactant amounts. One group may start with 2.0 grams of sodium carbonate, while another may start with 2.4 grams of sodium carbonate. I can also ask some groups to begin with certain amounts of calcium chloride and then have them calculate both the mass of the sodium carbonate needed and the ppt expected. Usually, 50ml of distilled water is enough to completely dissolve each reactant. I stress to the student that the less water they use, the less solution they will need to filter and the less time it will take.

After students have mixed their solutions in separate beakers, then I have them pour the contents of one beaker into the other. I have the students rinse the beaker with minimum amounts of distilled water. The students then pour the ppt formed through filter paper. The filter paper is in a funnel and the funnel is in an Erlenmeyer flask. Because I do not have a vacuum filtration system, it takes a long time for the filtering process to happen. This time allows them an opportunity to write up their lab report which will include a written procedure. I have them use a distilled water rinse to make sure all of the ppt is out of the beaker. The students will take the filter paper with the ppt in it, out of the funnel and lay it on paper in the lab area. It can take up to 3 days to completely dry. Because this is a slow process, it allows students to come in during lunch, after school and during other classes to complete the lab. This allows me an opportunity to have great conversations with them. When dry, the students will analyze % error and % completion for their results. I have the students do 2 trials.

Both sodium carbonate and calcium chloride are inexpensive chemicals. You will need distilled water to dissolve them into.

Materials needed: solid reactants, distilled water, chemical scoop, chemical weighing dishes, gram scale, Erlenmeyer flasks, funnels, filter paper,

Problems to solve:

1. You do not need ultrafine filter paper for this lab. I would encourage you to experiment with different kinds and choose what works best for you.

2. The filter paper can rip apart if a student tries to take it out of the funnel too soon.

3. If you have multiple sections of chemistry, you will need a lot of space to allow the ppts to dry. I had 4 sections with an average of 12 lab groups in each section. This made for 96 pieces of filter paper drying at the same time

4. Students will not get done filtering both trials during a class period unless you are on a block schedule. They will need to find time outside of class to complete this. If they wait too long, the ppt dries on the beaker.

5. Be prepared for some groups having to start over. Filter paper gets holes in it, stuff gets spilled, groups pour the solution in too fast and it goes over the sides of the filter paper and into the filtrate etc.

Reasons for doing this lab

1. It allows students to calculate the amount of ppt and then see immediate results

2. It allows me insight into areas of struggle with students. I can see frustration when things do not go correctly or when figures are incorrect

3. It makes students use time outside of class to complete an experiment

Balanced molecular equation

CaCl2 + Na2CO3 ________________________ 2NaCl + CaCO3

Balanced ionic equation

Ca++ + Cl- + Na+ + CO3-- _________________ Na+ + Cl- + CaCO3

Net ionic equation

Ca++ + CO3-- ___________________ CaCO3

Spectator ions Na+ and Cl-