- What mass and mole relationships are there in chemical reactions? In this experiment, you will find the mass of a sample of solid lead acetate and prepare a water solution of it. You will also find the mass of a zinc strip, place it in the solution, and observe its behavior. By finding the mass of the zinc strip at the close of the experiment, you will be able to investigate quantitatively any changes that occur.
- In keeping with chemical practice, we shall refer to the chemical substances by using appropriate symbols. Zinc is an element; it contains only one kind of atom. the symbol for zinc is Zn. Lead acetate is a compound with the formula Pb(CH3COO)2*3H2O. The formula identifies it by having the symbol Pb for the lead part and the symbols CH3COO for the for the acetate part. The group CH3COO , consisting of two carbon atoms, three hydrogen atoms, and two oxygen atoms, is often found in chemical compounds and has the name acetate. The subscript "2" following the parentheses indicates that two acetates combine with one lead atom. The "3H2O" in the formula indicates that there are three water molecules for every molecule of lead acetate in the crystals.
MATERIALS NEEDED:
SAFETY GOGGLES 
10-mL graduated cylinder Wash bottle 25-mL graduated cylinder 250-mL beaker Acetone 400-mL beaker Distilled water Stirring rod Zinc strip Small weighing dish Lead acetate Pb(CH3COO)2*3H2O Balance Dropper bottle with concentrated acetic acid PROCEDURE:
PRE-LAB
1. Record to 4 significant figures the Formula weight of the lead acetate Pb(CH3COO)2*3H2O, and the atomic weight of both zinc and lead, on the DATA TABLE. We use the term formula weight instead of molecular weight, because of the water which is not really bound, but part of the crystal structure.
DAY 1
2. Obtain a strip of zinc. If it has been used before, clean it with emery paper until a shiny surface is obtained.
3. To the nearest 0.01 gram, find the mass of the zinc strip, and record it on the DATA TABLE.
4. Find the mass of the 250-mL beaker to the nearest 0.01 gram. Record the mass of the beaker on the DATA TABLE.
5. Place your weighing dish on the balance, and using the propper procedure for the balance used, weigh out exactly 2.00 grams of lead acetate, Pb(CH3COO)2*3H2O . The mass is already recorded on the DATA TABLE.
IF YOU SHOULD TOUCH ANY OF THE CRYSTALS, BE SURE TO WASH YOUR HANDS.
6. Transfer the solid lead acetate to the 250-mL beaker. Add 200 mL of distilled water to the lead acetate, remember to use some of this water to rinse the weighing dish into the beaker. Swirl gently until all the solid dissolves. Add 2 or 3 drops of concentrated acetic acid, CH3COOH , to clarify the solution. REMEMBER: CONCENTRATED ACETIC ACID WILL BURN YOU.
7. Place the zinc strip in the beaker containing the lead acetate solution.
8. Observe for several minutes and record any changes that take place in the OBSERVATION SECTION. Carefully place the beaker in a safe place until tomorrow. DO NOT RAISE THE STRIP OUT OF THE SOLUTION.
DAY 2
9. Observe what has happened in the beaker. DO NOT RAISE THE ZINC STRIP UP OR THE LEAD WILL ADHERE TO IT AND YOU WILL NEVER GET IT OFF. Record your observations in the OBSERVATION SECTION.
10. Shake the lead off the zinc strip into the beaker by taping it corner to corner on the sides of the beaker. Raise the strip out a little and use your wash bottle to rinse into the beaker any lead that tends to adhere to the strip. Rinse the strip and set it on a paper towel to dry.
11. Let the lead settle in the beaker. Carefully decant the solution into the 400-mL beaker. Decant means to poor off liquid, leaving a solid behind. You are decanting into the 400-mL beaker so you wonÍt loose any lead because of an accident. Discard the liquid, you will only keep the lead precipitate in the 250-mL beaker.
12. Wash the lead residue with 10 mL of distilled water, measured with the 25-mL graduated cylinder. and carefully decant as before. Wash and decant at least 3 more times.
13. Obtain 10 mL of acetone in the 10-mL graduated cylinder, and wash and decant the lead sample with the acetone. This is to help the sample dry faster.
14. Place your 250-mL beaker with the lead sample, in a designated drying area. REMEMBER: The acetone fumes are flammable
DAY 3
15. Take your 250-mL beaker with the lead sample from the drying location, and set it on the table to cool. When it is cool, find the mass to the nearest 0.01 gram and record it on the DATA TABLE.
16. Find the mass of the zinc strip and record it on the DATA TABLE.
CALCULATIONS:
From the data in your data table:
17. Calculate the change in mass of the zinc strip by subtracting the mass of the strip after reaction from the mass of the strip before reaction. Record answer on the CALCULATION TABLE.
18. Calculate the mass of lead obtained by subtracting the mass of the beaker from the mass of the beaker and lead sample. Record answer on the CALCULATION TABLE.
19. Calculate the number of moles of zinc reacted. Remember that the mass of a substance divided by the mass per mole equals the number of moles. The change in mass of the zinc strip represents the mass of zinc reacted. The formula would be:
mass of zinc reacted (g)
Atomic weight of zinc (g / mole)20. Calculate the number of moles of lead produced This is the same calculation as in step 19, but using the data for the lead. Use the correct number of significant figures, and express your answer in SCIENTIFIC NOTATION. Record on the CALCULATION TABLE.
21. Determine the ratio:
moles Pb
moles Zn22. Record the mass of Pb(CH3COO)2*3H2O used in the experiment. This is just copied from the data table. Record in the CALCULATION TABLE.
23. Calculate the number of moles of Pb(CH3COO)2*3H2O used in the experiment, as you did in steps 19 and 20. Record your answer in SCIENTIFIC NOTATION on the CALCULATION TABLE.
24. Determine the ratio:
moles Pb
moles Pb(CH3COO)2*3H2O25. Using your ratios, write WHOLE NUMBER coefficients for the formula on the CALCULATION TABLE.
DATA TABLE:
Formula weight of lead acetate Pb(CH3COO)2*3H2O . __________
Atomic weight of zinc Zn . __________
Atomic weight of lead Pb . __________
Mass of zinc strip before reaction. __________ g
Mass of 250-mL beaker. __________ g
Mass of lead acetate sample. 2.00 g
Mass of 250-mL beaker and lead sample. __________ g
Mass of zinc strip after reaction. __________ g
OBSERVATION SECTION:
Observations of beginning reaction.
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Observations at end of reaction.
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CALCULATION TABLE:Change in mass of zinc strip. __________ g
Mass of lead obtained. __________ g
Number of moles of zinc reacted. __________________ mole
Number of moles of lead produced. __________________ mole
Ratio:
moles Pb : moles Zn ______ : __1__
Mass of lead acetate sample. __________ g
Number of moles of Pb(CH3COO)2*3H2O used. __________________ moleRatio:
moles Pb : moles Pb(CH3COO)2*3H2O ______ : __1__
___ mole of Zn (solid) + ___ mole of Pb(CH3COO)2 (solution) ----->
___ mole of Pb (solid)+ ___ mole of Zn(CH3COO)2 (solution)Because the water in the lead acetate crystals was not envolved in the reaction, it is left off of the formula.
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Back to Lab Dad's Laboratory
- In keeping with chemical practice, we shall refer to the chemical substances by using appropriate symbols. Zinc is an element; it contains only one kind of atom. the symbol for zinc is Zn. Lead acetate is a compound with the formula Pb(CH3COO)2*3H2O. The formula identifies it by having the symbol Pb for the lead part and the symbols CH3COO for the for the acetate part. The group CH3COO , consisting of two carbon atoms, three hydrogen atoms, and two oxygen atoms, is often found in chemical compounds and has the name acetate. The subscript "2" following the parentheses indicates that two acetates combine with one lead atom. The "3H2O" in the formula indicates that there are three water molecules for every molecule of lead acetate in the crystals.
BEHAVIOR OF SOLID ZINC IN AN AQUEOUS
SOLUTION OF LEAD ACETATE
INTRODUCTION: