- How is the chemical formula for a substance determined? In this experiment you will react solutions of lead nitrate, Pb(NO3)2 , and sodium iodide, NaI , of known concentration, 0.500 M. You will observe the results when various volumes of these solutions are mixed. From the relative volumes of the solutions used and their molar concentration, and from the masses of the precipitate formed, it will be possible to determine the number of moles of the substances involved and the formula for the precipitate.
REMEMBER, THESE CHEMICALS REACT WITH YOUR SKIN !!!
MATERIALS NEEDED:
*Lead nitrate solution 0.500M Nine,15ml-centrifuge tubes **Sodium iodide solution 0.500M Test tube rack 10-mL graduated Pipettes and pumps Stirring rod Centrifuge Rinse bottle Rack for all tubes to be dried Corks for centrifuge tubes ***Drying oven PREP:
* To prepare 100 mL of a 0.500 M solution of lead nitrate, Put about 50 mL of distilled water in a 100-mL volumetric flask. Add 16.56 g of lead nitrate Pb(NO3)2, rinsing any remaining particles into the flask with a rinse bottle of distilled water. Swirl to dissolve. Carefully add distilled water until the miniscus is exactly on the mark. This is enough for 2 runs of this lab.
** To prepare 100 mL of a 0.500 M solution of sodium iodide, Put about 50 mL of distilled water in a 100-mL volumetric flask. Add 7.49 g of sodium iodide NaI, rinsing any remaining particles into the flask with a rinse bottle of distilled water. Swirl to dissolve. Carefully add distilled water until the miniscus is exactly on the mark. This is enough for 2 runs of this lab.
*** The drying oven does not have to get hotter than just under boiling. An incubator turned all the way up is perfect.
PROCEDURE:
- If a class is doing the lab, each student or pair of students can be assigned one pair of solutions. If done alone, all nine pairs of solution must be done.
DAY 1
1. If you were given an assignment number, mark the assignment number on your data table. If you are doing the entire lab alone, you will need 9 data tables, one for each assignment number.
Table 1 gives the volumes that are to be carefully measured into a clean dry 15ml centrifuge tube. Record on the DATA TABLE your assignment number and the volumes of each of the two chemical solutions that correspond to your assigned number. The assignment numbers correspond to the volumes of the lead nitrate for ease in graphing later.
Table 1: Paired volumes of solutions.
Assignment0.500M
lead nitrate
(mL)0.500M
sodium iodide
(mL)1 1.0 9.0 2 2.0 8.0 3 3.0 7.0 4 4.0 6.0 5 5.0 5.0 6 6.0 4.0 7 7.0 3.0 8 8.0 2.0 9 9.0 1.0 2. Obtain a 15 ml centrifuge tube from the prep area. On the frosted area, write in pencil the assignment number. Find the mass of the centrifuge tube, without cork, to the nearest 0.01g and record it on the DATA TABLE.
3. One technique for measuring volumes of solution is with a pipette. We are using 10ml graduated pipettes. Read the proper procedure for using a pipette from the instructions with the pump, or from a textbook. Measure the required volumes of the solutions according to your assignment number, and put them both into the centrifuge tube. Be careful not to touch the centrifuge tube with the pipette, as this will contaminate the reagent bottles. Use a different pipette for each solution, and be careful not to mix up the pipettes.
Gently thump the tube to mix.4. Place the cork on the tube and put the tube in the rack with the other tubes to react overnight.
5. Record the FORMULA WEIGHT of both lead nitrate and the sodium iodide on the DATA TABLE.
CLEAN UP DAY 2
6. Remove your sample tube from the group rack and place it in your test tube rack at your station.
- We are going to use the centrifuge to spin down the precipitate, rather than filter. Follow the instructions that came with your centrifuge carefully to prevent equipment damage and possible injury. Tubes must be loaded in pairs. The shield pairs for a six-tube centrifuge are 1-4, 2-5, 3-6, and are used in that order. If there is an odd number of tubes, a balance tube (extra tube with 10 mL of water in it) must be used to even the load.
- Close the cover and make sure the latch is closed. Turn the rotary knob clockwise to the 5-minute dial mark.
- A bell will sound when the centrifuge power is off. Do not raise the cover until the centrifuge stops spinning.
7. Remove the cork from your sample and centrifuge according to the above instructions. When finished spinning, handle the tube carefully to avoid disturbing the precipitate.
8. Carefully pour off the liquid without loosing any precipitate.
9. Place the tube in your rack and add 5 mL of distilled water to wash the precipitate. Carefully break-up the packed precipitate by thumping the tube. If you chose to use a stirring rod, rinse the rod into the tube, so no precipitate is lost. Centrifuge as before. When finished spinning, pour off the liquid without loosing any precipitate.
10. Repeat step 9.
11. Place the tube in the group rack for drying. Put the rack of tubes into the desiccating oven for 24 hours.
CLEAN UP DAY 3
12. Remove the tube from the desiccating oven, and when cool, find the mass of the tube and precipitate to the nearest 0.01g. Record the mass on the DATA TABLE.
CALCULATIONS: RECORD ALL ANSWERS ON THE CALCULATION SHEET.
13. Obtain the mass of the precipitate by subtracting the mass of the centrifuge tube from the mass of the centrifuge tube and precipitate.
14. On the summary table, record the masses of precipitates obtained for assignments 1 through 9 by you and your classmates. If an assignment was done by more than one pair, record the average mass for that assignment.
15. Make a graph for plotting the information tabulated in step 14. Use the number of milliliters of lead nitrate solution (including 0) for the horizontal axis, and the number of grams of precipitate formed for the vertical axis. Draw the best fit straight lines through the points. There are 2 straight lines.
16. Explain the lines on your graph, and what they and the point where they cross mean.
17. Calculate the number of moles of lead nitrate and of sodium iodide used in each of the test assignments, and the ratio of moles of lead nitrate to moles of sodium iodide for each tube. Tabulate your answers on the table provided on the calculation sheet.
- To do this type of ratio, take the smallest number, give it the value 1 and record the 1 on it's side of the (:). Then divide the larger by the smaller number and round the answer to the nearest whole-number. Record that number on the proper side of the (:). Remember that the 1 will not be on the same side of the (:) every time, it is on the side that corresponds to the smallest number. The following is an example.
Table 2: Moles used, and ratios. Assignment
numberMoles of
Pb(NO3)2Moles of
NaIRatio: Moles 
Moles
Pb(NO3)2:NaI
1
0.00050
0.0045
1 : 9
2
0.0010
0.0040
1 : 4
18. On the top horizontal axis of your graph (from step 15), record the ratio of moles of Pb(NO3)2 to moles of NaI for each set of data.19. Refer to the graph and record the x and y value for the point that represents just enough of the lead nitrate solution to react with all of the sodium iodide and just enough of the sodium iodide to react with all of the lead nitrate. This is where your two lines cross.
20. What ratio of moles of Pb(NO3)2 to moles of NaI correspond to the point selected in question 19?
21. Use the ratio selected in question 20 to determine the formula of the precipitate. You may assume that the precipitate is composed of lead atoms and iodine atoms.
DATA TABLE:Assignment number. _____
Volume of 0.500M lead nitrate solution, Pb(NO3)2 . _______ mL
Volume of 0.500M sodium iodide solution, NaI . _______mL
Mass of centrifuge tube. _________ g
Formula weight of Pb(NO3)2 . ___________
Formula weight of NaI. ____________
Mass of centrifuge tube and precipitate. __________ gCALCULATION SHEET:
Mass of the precipitate for your assignment number. __________ gSummary table for masses of precipitates Assignment
numberMass of
precipitate (g)
1
2
3
4
5
6
7
8
9
Explain the lines on your graph, and what they and the point where they cross mean.
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Table of Moles used, and ratios. Assignment
numberMoles of
Pb(NO3)2Moles of
NaIRatio: Moles Moles
Pb(NO3)2:NaI1
:2
:3
:4
:5
:6
:7
:8
:9
:
Where the graph lines cross. x = ________ y = ________
Ratio of moles of Pb(NO3)2 to moles of NaI, where the graph lines cross. ___ : ___
Formula of precipitate. __________________
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Back to Lab Dad's Laboratory
- If a class is doing the lab, each student or pair of students can be assigned one pair of solutions. If done alone, all nine pairs of solution must be done.
PRODUCTION OF A PRECIPITATE
FROM SOLUTIONS OF
SODIUM IODIDE AND LEAD NITRATE
INTRODUCTION: