Limiting Reactant Lab – Exploring Molar Relationships



Limiting Reactant Lab – Exploring Molar Relationships

When two substances react in a chemical reaction, there is generally a limiting reactant. This reactant is the one that is consumed entirely and limits how far the reaction can proceed. The amount of product produced is dependant on the number of moles of this limiting reactant. The reactant that is not entirely consumed is called the reactant “in excess.”

In this activity you will react solid aluminum and a measured quantity of copper (II) chloride solution to determine which one is the limiting reactant. You will also determine the concentration (molarity) of the cupric chloride solution.

Procedure:

1. Put on your goggles.

2. Using a graduated cylinder, measure out 75mL of copper (II) chloride.

3. Carefully transfer this solution to the beaker provided.

4. Obtain a pre-cut piece of aluminum metal from your instructor. Find the mass of the aluminum and record the value in your data table.

5. Put the aluminum piece into the beaker with the copper solution (be careful not to make a splash!). You should begin to notice a lightening in the blue color of the copper solution and the creation of a red/brown solid. Stir with a glass stir rod on occasion to help remove the red/brown solid from the aluminum strip. Let the reaction sit until it reaches completion.

6. Take the remaining Aluminum out of the beaker with tongs or tweezers.

7. Holding the aluminum OVER THE SINK, rinse off as much of the brown solid as possible.

8. Then using a paper towel, dry off the aluminum as best you can and find it’s mass. Record the new mass in the data table.

9. Throw aluminum into the trash, clean all equipment, and wipe down your lab station.

Data Table:

|Mass of Aluminum Before Experiment | |

| |grams |

|Mass of Aluminum After Experiment | |

| |grams |

Analysis and Conclusion Questions:

1) Write the balanced chemical equation for this reaction.

2) Which reactant is the limiting reactant? What evidence do you have to support this?

3) What is the molarity of the copper (II) chloride? (hint: find moles of Al first, then moles of Cu…..you figure out the rest) Show all calculations.

4) A student forgets to dry the piece of aluminum before taking the final mass. Explain how this error will affect the calculated molarity of the copper (II) chloride solution. Clearly state whether the calculated molarity will be too high, too low, or unchanged by this error and include a mathematical justification for you answer.

LAB WRITE UP INSTRUCTIONS:

PreLAB – Name, date, title, purpose, flow chart/procedure, materials used

Copy data table

Post lab - Complete all analysis and conclusion questions. These are worth A LOT so don’t skip them.

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