LAB TITLE: Theoretical Yield, Limiting Reactant & Percent Yield

INTRODUCTION

The limiting reactant in a reaction is the chemical that determines how much product is formed from the chemical
reaction. The limiting reactant is completely used up in the reaction; it “limits” the amount of product that can form.
Ordinarily, in the laboratory, reactants are not mixed in exactly the ratio required for the reaction. Instead, an excess of
one reactant, usually the cheaper one, is used.

The excess reactant in a chemical reaction is not completely used up; some remains when the reaction stops (when the
limiting reactant is completely consumed). Some of the excess reactant remains because there is nothing with which it
can react.

In chemistry, actual yield, also referred to as chemical yield and reaction yield, is the amount of product obtained in a
chemical reaction.

 The theoretical yield is what is expected from the chemical equation before running the actual laboratory
procedure. This is obtained through stoichiometric calculations.
 Reactions rarely produce the predicted amount of product from the masses of reactants in the reaction. The
actual yield is the amount of product actually produced and measured during a reaction.
 The percent yield is a measure of the effectiveness of a chemical reaction, and is calculated by dividing the
amount of the obtained product (actual yield) by the theoretical yield and multiplying by 100.

Percent yield = _actual yield__ X 100

Theoretical yield

The theoretical yield of a chemical reaction is 100%, a value that is impossible to achieve due to limitations in
measurement accuracy.

Determine the theoretical yield of copper produced from the mass of iron used in the replacement reaction of iron plus
copper sulfate pentahydrate (CuSO4*5 H2O). Also determine the percent yield from the reaction and identify the
limiting and excess reactants.

MATERIALS:

 Evaporating dish (weigh empty, clean, dry & record)

 100 mL beaker (weigh empty, clean, dry & record)
 Approximately 0.50 gram of iron filings (Fe(s)) MM = 55.85 g/mol
 Approximately 5.00 grams of copper sulfate pentahydrate (CuSO4*5 H20) MM = 249.68 g/mol
 Wash bottle containing deionized water
 Ethanol
 Electronic balance
 Weighing dish
 Stirring rod
 Large beaker (for disposal)
 Tongs
 Dish soap
PROCEDURE:

Part 1: Reacting the chemicals

1. Measure and record the exact mass of a clean, dry 100 mL beaker.

2. Add approximately 5.00 g of copper (II) sulfate pentahydrate into the beaker. Record the exact combined mass. Add
approximately 35.0 mL H2O to the beaker and stir to dissolve the copper (II) sulfate pentahydrate. (You may need to
warm the solution slightly to get the solid to completely dissolve.)

3. Measure approximately 0.50 g of iron (Fe) metal filings onto the piece of weighing paper or weighing dish. Record the
exact mass of the iron filings.

4. While stirring the copper sulfate solution with the stirring rod, slowly add the iron filings to the copper (II) sulfate
solution. Do not add all the iron filings at once. Continue to stir until completely mixed. If small amounts of iron filings
are still floating, use a small amount of dish soap on the end of a dry stirring rod to make the filings settle to the bottom.

5. Allow the reaction mixture to stand, without stirring, for about five minutes to ensure complete reaction. The solid
copper metal will settle to the bottom of the dish.

Part 2: Collecting the copper

6. Use the stirring rod to decant (pour off) the liquid slowly into a larger beaker or container (for disposal). Be careful to
decant only the liquid. Put this waste in the LABELED waste container in the fume hood.

7. Add deionized water (5-10mL) to the copper solid left in the beaker and carefully stir/swirl the beaker to wash the
copper.

8. Decant the excess liquid from the beaker slowly and carefully into the disposal beaker leaving the solid behind.

9. Carefully weigh an empty, clean, dry evaporating dish. Record the mass.

10. Transfer the solid into the evaporating dish using small amounts of deionized water. Decant the excess water into
the disposal beaker.

11. Add ethanol (1-2 pipettes full) to the copper solid in the dish and carefully stir/swirl the dish to wash the copper.
Decant the excess liquid slowly and carefully into the disposal beaker. (this ethanol/water waste can go down the drain)

12. Place the evaporating dish containing the wet copper carefully on the hot plate. Use low heat to dry the copper. Do
not overheat the wet copper. The boiling point of ethanol is 78.1°C.

13. Once the copper is completely dry (it will move freely in the dish), remove the evaporating dish from the hot plate
using tongs and allow it to cool before weighing.

14. Measure and record the exact mass of the cooled evaporating dish and copper.

Cleanup and Disposal:

13. Make sure the hot plate is off and unplugged. Caution ** (Do not try to wrap the cord on the hot plate while it is
cooling.)

14. Place the dry copper in the designated waste container. Wet any residue that sticks to the evaporating dish and
wipe it out using a paper towel. Ethanol/water waste can go down the drain.

15. Wash and place all lab equipment back in its proper place.

16. Wash your hands thoroughly after all lab work and cleanup is complete.

17. Perform the necessary calculations to complete your data table.

DATA TABLE:

Mass of empty beaker

Mass of beaker and CuSO4*5 H20
Mass of CuSO4*5 H20 only
Mass of iron filings
Melting point of pure copper (look it up; give page #)
(CRC Handbook of Chemistry & Physics)--reference
Mass of clean dry evaporating dish
Mass of evaporating dish & dried copper
Mass of dried copper (actual yield)
Theoretical yield of copper in grams
Determine the limiting reactant
Determine the excess reactant
% yield of copper (actual/theoretical yield) X 100

Balance the equation, then use it to calculate theoretical yield:

_____CuSO4 * 5 H2O + _____Fe → Fe2 (SO4)3 + _____Cu + _____H2O

Determine the theoretical yield of copper in grams and show your work in the calculations section.

Determine the limiting and excess reactants (these are not amounts; reactants are species on the left in the equation).

Determine the percent yield and show your work in the calculations section.

SHOW your results to your instructor before you leave.