Knutsen's Chem 140 Lab: The Nine Solution Problem

Chem& 161
General Chemistry w/Lab I

Experiment: The Nine Solution Problem

Purpose
In this laboratory you will be provided with a set of nine solutions. Unfortunately, the labels have fallen off the bottles! It will be your task to identify the contents of each bottle. Part of your grade will be based on the correct identification of each.

Introduction
An important class of chemical reactions is double replacement, or metathesis, reactions. These occur when aqueous ionic compounds combine by trading or replacing ions. For example, in the following double replacement reaction:

3 AgNO₃ (aq) + Na₃PO₄ (aq) ——> Ag₃PO₄ (s) + 3 NaNO₃ (aq)

you can see that the ions have “traded places”. The cation from the first compound forms a compound with the anion from the second compound and vice versa. In a double replacement reaction one of the products is frequently insoluble and appears as a precipitate. In the case of the reaction above a precipitate of solid silver phosphate forms.

Because the reaction takes place in solution it is often appropriate to write the reaction in ionic or net ionic form. When ionic compounds are in solution, those compounds that are soluble will dissociate into ions; compounds that are insoluble do not dissociate. You will need to consult your text (p. 148 & Appendix C, p. A–13) or a reference book to determine which compounds are soluble and which are insoluble. In the double replacement reaction above, both of the reactants are soluble, while one of the products is insoluble and the other is soluble. Thus, the ionic equation for this reaction is best written as:

3 Ag⁺ (aq) + 3 NO₃^- (aq) + 3 Na⁺ (aq) + PO₄^-3 (aq) ——> Ag₃PO₄ (s) + 3 Na⁺ (aq) + 3 NO₃^- (aq)

In this example, the compounds which are soluble in water (AgNO₃, Na₃PO₄ and NaNO₃) have been shown dissociated into ions. This is how these compounds would actually exist in solution.

The net ionic equation is similar, but removes clutter in the reaction by “canceling out” those substances which appear on both sides of the equation. These are called spectator ions; they have not undergone any chemical change. Thus, the net ionic equation for this reaction would be:

3 Ag⁺ (aq) + PO₄^-3 (aq) ——> Ag₃PO₄ (s)

This equation shows only those substances which are undergoing chemical change. The spectator ions, Na⁺ and NO₃^-, have been removed.

In this lab you will work with the following nine solutions:

AgNO₃	Na₂CO₃	NaBr
NH₄Cl	Ca(NO₃) ₂	H₂SO₄
NaOH	BaCl₂	H₂O

You are to mix each solution with each of the others, and determine whether the ions have combined to form new substances. You will be able to determine if a reaction has occurred if you observe one of the following:

The solution becomes cloudy/opaque, or a solid is observed settling to the bottom, indicating that a precipitate has formed. You can use the solubility rules to predict which solutions will form a precipitate when mixed.
A gas is formed. The primary method for detecting these gases is by odor or the appearance of bubbles, both of which can be difficult to observe. You may or may not see bubbles forming, or detect the color or odor of a gas. Refer to your text (p. 156) for help in predicting which of these reactions will produce gases.

Procedure
In this laboratory you will work with your lab partner to complete the following:

Obtain a well plate and nine plastic droppers from the lab cart. Be certain that all of your equipment is clean. Any contamination will affect your results.
Use tape to label each of the plastic droppers with numbers 1–9. Fill each dropper with the appropriate solution and place them on a paper towel on your desk.
Combine all of the solutions in pairs of two, i.e., 1+2, 1+3, etc., and record the results in your lab notebook. Obviously you do not have to combine a solution with itself! If no reaction occurs, write NR.
Clean the well plate and droppers thoroughly and return to the lab cart.
Use the information from your text and the results from your experiments to identify the nine solutions.

Analysis and Report
Be sure to include the following in your laboratory report:

Your data, showing the results when every possible combination of solutions is mixed. A chart or grid is a clear way to present this information.
Your identification of the nine solutions, along with a complete explanation of how you made each identification.
The molecular and net ionic equations for every reaction that produced an observable result. (Omit duplicate reactions.)

Your identification of the nine solutions must follow from your experimental data. You will lose points if you give results which are not supported by your data and reasoning.

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