#4538. Determining the Identity of Unknows by using Solubility Rule

Science, level: Senior
Posted Wed Mar 30 21:05:38 PDT 2011 by Kelly Timson (Kelly Timson).
GBS, Glenview, IL
Concepts Taught: Chemistry

Trouble in the Chemistry Department: Utilizing Solubility Rules to Discover the Identity of 4 Unknown Solutions

Target Audience
1st year high school chemistry students

Concept / Topic To Teach
Solubility Rules, Identifying Unknowns, Double Replacement/Precipitation Reactions

Objectives
Students will be able to:
• utilize the solubility rules
• write double replacement reactions and predict in which cases a reaction should occur (when a precipitate forms).
• correctly identify 4 unknown aqueous solutions

Required Materials
• Lab Handout (see attached)
• Dropper bottles of each of the following 0.10 M solutions: CuSO4, KNO3, NaCl, AgNO3, NaOH.
• Well Plates

Anticipatory Set
• Begin with a warm up asking students to use their solubility rules to predict whether each of the following substances is soluble or insoluble in water: AgOH, Na2SO4, ca(NO3)2
• Next, have students complete the following double replacement reactions and identify the precipitate (if any)
NaCl (aq) + AgNO3 (aq) 

NaOH (aq) + KNO3 (aq) 

• Finally, pose the following questions to the students and lead them in a discussion of the answers: What would you expect to observe if a solution of NaCl was mixed with a solution of AgNO3? How about a solution of NaOH mixed with a solution of KNO3?

Laboratory Work
Give students the lab handout (see attached). Have them go into the lab and collect data. The data collection will be quick. The majority of the time will be spent trying to determine which solution is which.

Sample Data
Sample data is based on Solution 1 being AgNO3, Solution 2 being KNO3, Solution 3 being NaOH, and Solution 4 being NaCl.

Solution CuSO4 1 2 3 4
1 No reaction X No reaction Ppt formed Ppt formed
2 No reaction No reaction X No reaction No reaction
3 Ppt formed Ppt formed No reaction X No reaction
4 No reaction Ppt formed No reaction No reaction X

Sample Conclusion
Here is a brief description of how the students should come up with the identity of the unknowns:

Solution 3 is NaOH because it is the only one of the four that will form a precipitate with CuSO4.

Solution 2 is KNO3 because it does not form a precipitate with anything.

Solution 1 is AgNO3 because it forms a precipitate with Solution 3, which we have already determined is NaOH.

Solution 4 is NaCl because it forms a precipitate with AgNO3, which is Solution 1.

Assessment
Collect and evaluate each student's data and conclusion.

Give a quiz in a couple days with sample data and ask students to determine unknown identity based on solubility rules.

Trouble in the Chemistry Department
Introduction
There is a major problem in the chemical stockroom. Upon arriving back to school after the summer, a teacher found a box containing a bunch of unlabeled bottles and 4 different labels at the bottom of the box. The labels must have fallen off due to the humidity.

By using your solubility rules, help us figure out which numbered solution matches up with the following chemicals:
AgNO3,NaCl, KNO3, NaOH

You will be given a labeled solution of CuSO4 to help you with this task.

Procedure
You will be provided with a labeled solution of CuSO4 as well as the 4 unknowns in bottles labeled 1,2,3,4. You will also be provided with a well plate. Mix each solution with each of the other solutions, observe, and record observations. There is no need to mix a solution with itself.

Data
Record all relevant data in a neat and organized table.

Conclusion
Be sure to address the following in your conclusion:
• The identity of the solution in each numbered bottle
• A detailed description of how you figured out which solution was which
• Complete balanced equations for all reactions that produced a precipitate