Student s Name: Date : Precipitation and gravimetric analysis of lead Iodide

Transcription

1 Student s Name: Date : Background Precipitation and gravimetric analysis of lead Iodide In this experiment, you will obtain quantitative results for the reaction between lead (II) nitrate and sodium iodide solutions. This reaction produces a bright yellow precipitate of lead (II) iodide, PbI 2. This experiment is designed as a whole class activity. You will be assigned group numbers. Each group will use different sets of masses. In each set the mass of lead (II) nitrate will be the same but sodium iodide masses will be different. The masses of lead (II) iodide produced will be determined by weighing after drying. Ultimately, you will individually produce a table with the class results then calculate the moles of reactants and the precipitated product. Equipment and chemicals lead (II) nitrate Pb(NO 3 ) 2 sodium iodide (NaI) filter funnel retort stand and ring clamp filter paper (Whatman N o 1 - one 12.5 cm piece) 2 x glass rods (one fitted with rubber policeman) analytical balance oven set to 70 o C ethanol (5 ml approx) distilled or reverse osmosis (RO) water Safety Lead salts are poisonous and must be handled with care. Make sure that the lead (II) nitrate does not come into contact with your skin. International safety card information lead (II) nitrate - sodium iodide - STAWA8.DOC Rob Namestnik Page 1

2 Procedure 1. Get your group number from the teacher and write it down here: 2. Weigh out the amounts of Pb(NO 3 ) and NaI required into separate 100 ml beakers using the masses designated for your group number on page Add about 35 ml of RO water to both beakers and stir with a stirring rod to aid help them dissolve. Do NOT take the rods out. You might have to heat the solutions on the hotplate to help them dissolve. 4. Carefully add the lead (II) nitrate solution to the sodium iodide solution. Rinse the empty beaker 3 times into the sodium iodide beaker with about 5 ml of RO water to make sure all the lead (II) nitrate is transferred. 5. Stir the mixture and allow it to stand for 5 to 10 minutes. Leave the glass rod in the beaker. 6. Weigh a filter paper and record its mass using the analytical balance. Write the mass on the data sheet on page Fold the filter paper and place it in a filter funnel supported by a retort stand and ring clamp. Moisten the filter paper with a few drops of RO water to help it sit in the funnel. 8. Carefully filter the solution containing the precipitate, ensuring you make a quantitative transfer (without spilling a drop) to the filter paper. Use the glass rod to direct the precipitate into the paper as in the diagram below: 9. Rinse the walls of the beaker with about 5 ml of RO water and add the rinsings to the filter paper. If the precipitate adheres (sticks) to the sides of the beaker, use a glass rod with a rubber policeman to scrape the lead (II) iodide. Repeat the rinsing procedure with two more 5 ml volumes of distilled water. STAWA8.DOC Rob Namestnik Page 2

3 10. After draining, wash the precipitate with about 5 ml of ethanol and allow this to drain away. 11. Carefully remove the filter paper and place it in a labelled 100 ml beaker. 12. Place the beaker in a drying oven at 110 o C and leave it for about one hour. 13. Re-weigh the filter paper and precipitate and record the mass. Enter the mass in the data sheet. 13. Wash all glassware and place in the drying rack. 14. Dispose of the precipitate in the bin and wipe your work area bench clean. Add your weighing results to the table (on the white board or over head projector) Return to your seat and begin processing the results. Data Sheet Group Mass Pb(NO 3 ) 2 Mass NaI MASS Analytical Balance ERROR Mass of filter paper + PbI 2 (s) ± g Mass of filter paper ± g Mass of PbI 2 (s) ± g Results Table Group Number Pb(NO 3 ) 2 NaI PbI 2 Moles Pb(NO 3 ) 2 Moles NaI x x Moles PbI 2 STAWA8.DOC Rob Namestnik Page 3

4 Processing of Results 1. Write a balanced equation for the reaction. 2. Calculate the mass of PbI 2 produced and enter this into your results table. Write the other groups results into your table. You may use a computer to produce an MS Excel spreadsheet for Question 3 (Enter formulas to allow the spreadsheet to calculate moles where needed) 3. Convert the masses given in the table into moles and write them in the results table. You may use a computer to produce an MS Excel spreadsheet for Question 4 4. Plot a graph in which the number of moles of potassium iodide is plotted along the x-axis and the number of moles of lead (II) iodide is plotted along the y-axis. 5. Look at the graph closely and describe the shape. Why is it this shape? 6. For group numbers 1 to 5 a) which reactant is present in excess? b) which is the limiting reactant? (Explain this in terms of the equation) 7. For groups 7 to 10 a) which reactant is present in excess? b) which is the limiting reactant? (Explain this in terms of the equation) STAWA8.DOC Rob Namestnik Page 4

5 8. What is the mole ratio of Pb(NO 3 ) 2 to KI for group number 6? Comment on how important this ratio is in relation to your graph and the reaction equation. 9. Use the website links provided under the safety notes on page 1 to find safety information for the following: a) Describe in your own words, the dangers associated with using lead (II) nitrate. b) How should lead (II) nitrate be safely disposed in the laboratory? c) Describe in your own words, the dangers associated with using sodium iodide. STAWA8.DOC Rob Namestnik Page 5

6 As an option there is an excel spreadsheet at the website in the Leeming SHS link for STAWA Experiment 8 that will allow you to input data from the PbI 2 weighings. After recording the results from all the groups, type them into the correct column in the spreadsheet and compare the moles of PbI 2 produced. Mole of reactants and product calculator Group Number Pb(NO 3 ) 2 NaI PbI 2 Moles Moles Moles Pb(NO 3 ) 2 NaI PbI M Pb(NO 3 ) 2 = M NaI = M PbI 2 = 331 g/mol 150 g/mol g/mol n(pb(no 3 ) 2 ) = m(pb(no 3 ) 2 ) /M(Pb(NO 3 ) 2 ) n(nai) = m(nai)/m(nai) n(pbi 2 ) = m(pbi 2 )/M(PbI 2 ) STAWA8.DOC Rob Namestnik Page 6