This week in lab, you'll be using the chemical tests you observed last week to separate (and identify) the metal cations in an unknown mixture of cations. It's probably better to think about this as a separation rather than just an identification because you will be given 1 sample and through a series of chemical (or physical) steps you will end up with up to 5 different metals in 5 different container. Some tips:
1. Flow chart - You have to organize your procedure to do well. The logical way to do this is with a flow chart for this type of a problem... a sequence of steps with decision points and branches along the way. You might not be a "flow chart person", but it makes it MUCH easier to follow through on a logical set of tests if you become a bit of a flow chart person for this experiment. Here's an example of a flow chart for separating and identifying anions if you're looking for an example: http://chemlab.truman.edu/chemlab_backup/CHEM131Labs/QualFiles/Figure2.gif
2. Assume you have all 5 metals - OK, there's not much chance that you'll get the sample that has all 5 metals, but you might. Even if you don't, design your flow chart and approach so that it will work for any possible unknown in this experiment.
3. The first step is the key! - Any step in which you are potentially making 2 or more precipitates represents a potential problem because you need to be able to separate those solids from one another. If you can't separate the solids from one another, it's not a useful test. For example, chromate makes a BUNCH of precipitates with the metals we are using, but there's no way (in our list of chemical tests) to separate those solids from one another. Adding chromate would be a horrible first step in your flow chart, but it might be handy later on when you only have to test for the presence of a single metal cation.
4. Positive tests - You need to have a positive confirmation test for all the metals, do not try to infer the presence or absence of a specific metal cation based upon negative results.
5. Chemical equations - If you come to lab prepared with a good plan, the "wet" part of this lab will not take three hours. This is a lab report experiment and as part of your lab report, you will have to write out balanced chemical equations for ALL of the tests you performed in the first week of the experiment. {quit rolling your eyes, it's not that hard and it's good practice!} Take advantage of your time in lab to make sure you understand the chemical equations and ask your instructor and lab assistant for feedback.
Good luck on your unknowns. One final note on your flow chart: this is one of those experiments where there is not a single correct answer. There are probably half a dozen or more variations on the "correct" flow chart that all work well.
Showing posts with label qualitative. Show all posts
Showing posts with label qualitative. Show all posts
2012-04-04
Qualitative Analysis of Metal Cations - Week 2
This week in lab you're going to be analyzing an unknown mixture of the metal cations you studied last week. To do this, you need to look over the tests you did last week and find a way to sequentially use some or all of those tests to separate the cations. This is NOT a "run all the tests and figure it out later" experiment, you have to have a plan.
To develop your plan, assume you are starting with a mixture of all 5 cations, and you want to separate them into 5 different containers. The key to developing a good flow chart is the ability to separate solids from each other when multiple things precipitate. For example, if the first step of your flow chart is "add chromate", you will precipitate all 5 cations as their chromate salts. You have no way to separate these solids from each other, so this would be a VERY bad first step. Look over some of the multi-step tests you did last week. If adding some reagent causes 2 or 3 of the cations to form precipitate, and you have a way to separate those precipitates from each other (with another step in the multi-step test you performed last week), then that might be a good place to start.
As an example, what if you had a mixture of NaCl, NaNO3, sand, and sawdust. How could you separate them? If you added water, the NaCl and NaNO3 would dissolve, the sand would sink, and the sawdust would float. That accomplishes some of the separation, but what about the dissolved salts? If there was a reagent you could add to make chloride ions form a precipitate, like maybe Pb2+(aq), you would be able to separate the chloride from the nitrate.
{OK, picky people in the crowd, that doesn't exactly separate "NaCl" from "NaNO3", but it illustrates the point!}
Given the tests you performed, there are a few different flow charts that will work to separate the 5 cations you're working with, so if you have something that's a little different from someone else, that's OK. It would be great if you compared your flowchart to someone else's and had a discussion about the similarities and differences, it might lead you BOTH to make better flowcharts/plans.
To develop your plan, assume you are starting with a mixture of all 5 cations, and you want to separate them into 5 different containers. The key to developing a good flow chart is the ability to separate solids from each other when multiple things precipitate. For example, if the first step of your flow chart is "add chromate", you will precipitate all 5 cations as their chromate salts. You have no way to separate these solids from each other, so this would be a VERY bad first step. Look over some of the multi-step tests you did last week. If adding some reagent causes 2 or 3 of the cations to form precipitate, and you have a way to separate those precipitates from each other (with another step in the multi-step test you performed last week), then that might be a good place to start.
As an example, what if you had a mixture of NaCl, NaNO3, sand, and sawdust. How could you separate them? If you added water, the NaCl and NaNO3 would dissolve, the sand would sink, and the sawdust would float. That accomplishes some of the separation, but what about the dissolved salts? If there was a reagent you could add to make chloride ions form a precipitate, like maybe Pb2+(aq), you would be able to separate the chloride from the nitrate.
{OK, picky people in the crowd, that doesn't exactly separate "NaCl" from "NaNO3", but it illustrates the point!}
Given the tests you performed, there are a few different flow charts that will work to separate the 5 cations you're working with, so if you have something that's a little different from someone else, that's OK. It would be great if you compared your flowchart to someone else's and had a discussion about the similarities and differences, it might lead you BOTH to make better flowcharts/plans.
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