Lesson 10: Experiment Preview
In this section I'll give you some information about the experiment that you'll be doing. We call it the five solutions experiment. For this experiment you will be given a list of 5 numbered bottles, and a list of 5 chemical formulas. All you have to do is match the formulas of the compounds in solution with the numbers of the bottles they are in.
Example
As an example of how you would go about doing that kind of thing, let's work through a simpler 3-solution experiment, (also shown in example 6 in your workbook). In this 3-solution experiment we're given bottle numbers 1, 2 and 3, and we know that H2SO4, CaCl2 and NaCl are in solution in those three bottles.
First, I would test the solutions with pH paper. From that test , let's say we found that solution #1 has a pH of 6; solution #2 has a pH of 2; and solution #3 has a pH of 7. From this information we can identify that solution #2 is distinctly acidic because its pH is much less than 7. Looking at the formulas, you can see that H2SO4 is the only one with hydrogen, oxygen and a nonmetal. Therefore, it is the formula of the acid. So solution #2 must be H2SO4.
Now that we've identified one of the three, we have a handle we can work with. Let's take a look at the reactions that take place when the solutions are mixed. Let's say that this is what we find. When solution #1 and solution #2 are mixed together, there is no reaction. When solution #1 and solution #3 are added together, there is no reaction. But when solution #2 is added to solution #3, there is a white precipitate. |
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no precipitate
Keep in mind that we already know that solution #2 is H2SO4. Working with the formulas, we can use the idea that precipitation reactions are a type of double displacement reaction. Solution #3 might be CaCl2 or NaCl. The products of these reactions are HCl + CaSO4 and HCl + Na2SO4. We know that HCl is soluble because the solubility rules say that all chlorides are soluble except AgCl, PbCl2 and Hg2Cl2. Therefore, the precipitate that formed must be one of the other compounds. If you look at the solubility rules, you'll see that the only insoluble sulfates are those of barium, calcium and lead. Therefore, the precipitate must be calcium sulfate. Therefore, solution #3 must have been calcium chloride, because that provided the calcium that reacted with the sulfate to form the white precipitate. That also tells us that solution #1 must be sodium chloride. |
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So that's how you would go about solving a three-solution experiment, given certain kinds of data. You should try the 6-solutions problem given at the end of the experimental procedure for this week. You'll need to complete it and have the lab instructor check it before you'll be allowed to start on the experiment.
A Little More Advice
When doing this experiment, the kinds of data that you look for are such things as the pH and precipitates.
You use pH paper so that you can identify acids and bases. Be careful when trying to match up the formulas of acids and bases with the pH of the solutions. Sometimes salts can be acidic or basic. Sometimes a bit of acid is added to a solution to help dissolve a salt and stabilize the solution.
You also look for precipitation reactions. You mix solutions together to see which ones precipitate, and that together with the solubility rules tells you about what some of the different combinations can be.
Also look for the formation of gases. Gas bubbles would indicate that you have a bicarbonate or a carbonate as one of the solutions if it reacts with acid to give off carbon dioxide gas. You also "look" for the odor of ammonia when things are added to bases, because a base added to ammonium ion will give ammonium hydroxide, which evaporates as NH3 and has the characteristic smell of ammonia.
Take a look at the introduction and instructions to the experiment in your workbook. There is quite a bit of information before you actually get to the experiment itself. Please read through all of the instructions for this experiment. There's the introduction which tells you about the nature of the experiment, lab hints, a set of solubility rules tailored to this experiment, another example, and a practice problem.
It's very important that you pay attention to what you are doing and record your data carefully. You can get yourself rreally fouled up if you did something different from you thought you did. For example, if you thought you added solution 2 to solution 8, but you really picked up solution 3 and added it to 8, you're going to have problems identifying what's in solution 2. So be careful and pay attention to the hints that are given.