Lab Activity __: Gravimetric Stoichiometry I Introduction

Stoichiometry involves making predictions about how much of one substance should react or be produced relative to a given amount of another species involved in a reaction.

In this activity, you will calculate the amount of silver that should be produced (theoretical yield) from a measured amount of a reactant and compare it to the amount of silver that is actually produced (actual yield). The predicted amount is based on stoichiometric calculations. The actual amount is based on careful mass measurements.

Careful measurements and attention to detail are very important in this activity. The activity will be carried out over a period of three days. You will need access to the lab for each of those days.

Problem

How does the mass of a product relate to the mass of a reactant in a chemical reaction?

Pre-Lab Assignment

Write a balanced chemical equation (including state of matter subscripts) for the reaction between silver nitrate solution and copper metal. (Hint: One product will consist in part of copper(II) ions.)

Materials

per pair of students: small container of silver nitrate crystals 20 cm length of heavy gauge pure copper wire piece of steel wool piece of filter paper (10 cm disc) 250 mL beaker 500 mL beaker filtration apparatus (ring stand, ring clamp, funnel) watch glass wash bottle containing distilled water centigram balance stirring rod with rubber tip scoopula paper towels

Safety

1. Wear safety glasses throughout this activity.2. Dispose of chemicals as directed in the procedure.3. Copper(II) ions are toxic. Wash and thoroughly rinse your hands if you come in

contact with the copper solution produced in this activity.4. Wash your hands at the end of each laboratory session.

Procedure

Day 1

1. Measure and record the mass of a 250 mL beaker.2. Preset the balance to include the mass of the beaker plus 2.00 g of silver nitrate

crystals.3. Add silver nitrate crystals to the beaker until the balance zeros4. Add approximately 125 mL of distilled water to the beaker. Stir the mixture with

the glass end of the stirring rod until the silver nitrate is completely dissolved.5. As the stirring rod is removed from the beaker, rinse it with water from the wash

bottle.6. Clean the copper wire with the steel wool to remove any oxides or coatings.7. Use the centigram balance to determine the mass of the coiled copper wire.

Record the mass of the wire.8. Wrap the copper wire around a pencil or the stirring rod to form a coil with a

handle.9. Place the copper wire in the silver nitrate solution.10. Record visual observations for the first few minutes of the reaction.11. Cover the beaker with the watch glass, and set aside in a safe place where it will

not be disturbed for a full day.

Day 2

1. Record visual observations describing the changes that have occurred in thecolour of the solution, the appearance of the copper wire, and the formation of anew solid.

2. Gently shake the solid crystals from the copper wire. Use the wash bottle and therubber tipped stirring rod to remove any stubborn crystals from the wire.

3. Remove the wire from the solution and dry it completely with paper towel.4. When you are sure the wire is completely dry, find and record its mass.5. Fold the filter paper in half, make a crease, fold it again to make a quarter disc,

and make a second crease.6. Find and record the mass of a piece of the filter paper.7. Set up the filtration apparatus using the funnel, ring and ring stand, and the 500

mL beaker.8. Let the silver metal settle and decant as much of the solution as you can into the

funnel.9. Use the rubber tip of the stirring rod to transfer the silver metal onto the filter

paper in the funnel.10. Use the wash bottle to rinse any remaining silver from the beaker and into the

funnel.11. Wash the silver crystals several times to remove any soluble substances like

copper(II) ions and nitrate ions from the residue.12. Remove the filter paper cone from the funnel by pressing your thumb against the

side that has three layers of filter paper in a gentle upwards motion13. Place the filter and the residue on a clean watch glass and unfold.14. Leave the paper containing the silver in a safe place to dry for one or two days.15. Dispose of the filtrate in the 500 mL beaker by pouring it down a sink and

flushing the sink with plenty of water.

Day 3

1. When the residue (i.e. the silver) is dry, find and record the mass of the filterpaper and the silver. You should repeat the mass measurement later on in theday to ensure that it is constant. If there is a change, then the silver is notcompletely dry. Let it dry overnight and repeat these steps the next day.

2. Inspect the paper for blue rings or any other signs of copper and record yourobservations of the appearance of the dried paper.

3. Once you have made your final observations and mass measurements, transferthe silver to a vial or a bottle labeled "Silver Metal". Have your supervisor writehis/her name and the date on the label.

4. Dispose of the filter paper by placing it in a waste basket.

Results

Prepare a table of your own design to summarize the observations recorded during the activity. You will need to use the information in your table to complete the analysis items below.

Analysis 1. By subtraction, calculate the actual mass of silver metal produced. This is the

actual yield. 2. Use stoichiometry to calculate the mass of silver that should have been produced

from the reaction of 2.00 g of silver nitrate crystals. This is the theoretical yield. 3. Percent yield calculations and interpretation.

a. Calculate the percent yield of silver using this equation:

b. If the percent yield is less than 100%, suggest at least two specificreasons for the lower than expected yield of silver metal.

c. If the percent yield is greater than 100%, suggest at least two specificreasons for the higher than expected yield of silver metal.

4. By subtraction, calculate the mass of copper metal that actually reacted.5. Use stoichiometry to calculate the mass of copper metal that should have

reacted with 2.00 g of silver nitrate crystals.

Extension

Filtration Technique Items: 1. Draw and label the filtration apparatus you used in this activity.2. What is an appropriate height for solution in the filter funnel? 3. What does "decanting" mean?4. Why does the precipitate or residue trapped by a filter have to be rinsed?5. What might cause the filter paper to appear blue after it dries in this activity?

Chemical Reaction Items: 1. What evidence suggests that all of the silver nitrate was reacted?2. Which atoms became cations in this reaction?3. Which cations became atoms in this reaction?4. Which ions resulted in the blue colour seen in the final solution?5. Why was the stirring rod rinsed in step 5 of the Day 1 procedure?