ACP Chemistry

Solution Stoichiometry Lab

 

Objective:

Apply stoichiometry and the idea of a limiting reactant to a reaction in solution.

 

Skills:

 

Background:

The study of mass relationships in chemical reactions is called stoichiometry. All problems in stoichiometry involve mole-mole conversions. Such calculations are needed to determine both the quantities of reactants that are required for a chemical reaction, and the theoretical yield of the reaction.

The reaction you will carry out in lab is a double replacement reaction between sodium carbonate and calcium chloride, in which a precipitate will form. You will isolate the precipitate by a process called filtration.

In reactions, one reactant is often used in excess in comparison to the other. The excess reactant will not react completely. The reactant that is not used in excess (it gets completely used up in the reaction) is referred to as the limiting reactant. The amount of product formed is dependent upon the amount of limiting reactant present.

 

Pre-lab (due at the beginning of class):

Answer all these questions below on the front side of DATA TABLE. 

PRINT OFF DATA TABLE HERE

  1. Write the balanced equation for the reaction of an aqueous solution of sodium carbonate with an aqueous solution of calcium chloride.  Note that you will have to determine which of the two compounds formed is the precipitate. Include the symbols (aq) for an aqueous solution and (ppt) for the solid precipitate.

  1. Use the Molarity formula (M = # moles / L solution) and dimensional analysis to determine how many grams of sodium carbonate you need to make 50.0 mL of a 0.700 M solution.  Then, write this amount into the blank space in your procedure.

  2. Use the Molarity formula (M = # moles / L solution) and dimensional analysis to determine how many grams of calcium chloride needed to make 100.0 mL of a 0.700 M solution.

  3. If 50.0 mL of a 0.700 M solution of Na2CO3 reacts with 60.0 mL of a 0.500 M solution of CaCl2, what is the limiting reactant?  Show both dimensional analysis setups and then clearly label the LIMITING REACTANT (LR) and the EXCESS REACTANT (ER).  Put a box or circle around the theoretical amount of precipitate that you should get in this reaction.

Materials:

 

Procedure:

1. Using a 50 mL volumetric flask, prepare 50.0 mL of 0.700 M Na2CO3 solution by following your recipe using the amount you have indicated in the Pre-lab work. 

“To make 50.0 mL of a 0.700 M solution of sodium carbonate: In a 50 mL volumetric flask, add _______________ g of sodium carbonate powder."

HOW TO USE A VOLUMETRIC FLASK: Add solute to volumetric.  Fill your volumetric flask about half full of distilled water.  Now, holding the volumetric flask high in the neck, swirl vigorously so that the base of the volumetric is going around and around.  (But, don’t let any solution escape out the top.) This process should dissolve a lot of the solute.  Add some more distilled water.  Swirl again.  When almost all the solute is dissolved, carefully fill up the volumetric to the etch mark on the neck. DON'T go above the etch mark or you will have to dump everything and start over.

2. You want to have a very, very clean graduated cylinder.  So, rinse out a 100 mL graduated cylinder with tap water and then final rinse with distilled water.

3. You want to have a very, very clean beaker.  So, rinse out a 250 mL beaker with tap water and then final rinse with distilled water.

4. Pour the contents of the 50.0 mL of .700 M Na2CO3 solution from the volumetric flask into the CLEAN 250 mL beaker.

5. Measure out EXACTLY 60.0 mL of 0.500 M CaCl2 solution (the teacher has made this solution for you) using the CLEAN 100 mL graduated cylinder. Pour that into the beaker with the solution of NaCO3.

6. Do not use more than 60.0 mL of the calcium chloride solution and do not use more than 50.0 mL of the sodium carbonate solution. 

7. Stir with stirring rod to mix the solutions well and then let the precipitate that has formed settle for 1 minute.

8. Now stir up the solution again so that the white precipitate is thoroughly mixed - your beaker should look all "milky."

9. Get a piece of filter paper.  In PENCIL, write lab partners names and block number.  Then, find the mass of the filter paper and record it here: _____________________ g

10. Fold filter paper into quarters to make a funnel shape (fold into quarters and then open up one side).  Place into funnel which sits on the neck of an Erlenmeyer flask.  Using the wash bottle, thoroughly wet down the filter paper so that it "sticks" to the side of the funnel.

11. Pour part of the solution from the beaker into the filter paper funnel BEING CAREFUL NOT TO HAVE THE SOLUTION GO UP OVER THE TOP EDGE OF THE FILTER PAPER.

12. Continue this process until all of the solution from the beaker has been filtered.  There will probably be some precipitate still in the beaker.  Squirt some distilled water into the beaker, swirl around and then pour it through the filter paper.

13. After the solution has filtered through, carefully remove the filter paper and keep in the "cone shape" so that when the precipitate dries, you won't lose any of the precipitate off the filter paper.  Place on a clean tray and let dry for at least 24 hours or until the paper and the precipitate is completely dry. 

14. Dispose of the filtrate (the clear solution in the bottom of the Erlenmeyer flask – the solution that went through the filter paper) in the sink.  Thoroughly rinse out all glassware and return all equipment to the front of the lab room.

14. When the filter paper and precipitate is COMPLETELY dry, record the mass of the filter paper + precipitate here: _____________________ g

Analysis:

You will answer these questions on the back of your Data Table.

  1. After the filter paper has dried, calculate the actual yield (in grams) of the precipitate.  SHOW ALL WORK HERE.  Remember, always include units. 
  1. Calculate the PERCENT YIELD for the precipitate formed.  SHOW ALL WORK HERE.  Remember, always include units.
  2. Percent yields for this reaction typically run above 80%. Was your percent yield high or low?
    List at least two factors that could have affected your percent yield (increased or decreased it, as the case may be). Note that there may be factors that work against each other. 

Source: http://faculty.northseattle.edu/tfurutani/che101_2008/lab4.pdf

 

 

 

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