Finding the percentage of Nickel in an unknown compound.

LAB 3

Purpose: Finding the percentage of Nickel in an unknown compound.
Objectives: To separate and weigh nickel from a sample that contains an unknown amount of nickel and to use the mole method to determine the amount of nickel in the unknown sample.
Background:

Analytical chemistry can be simply divided into two areas. Qualitative analysis and quantitative analysis. The two words “qualitative”, means to find out if a specific substance is present in a soil sample, and “quantitative” means to determine how much of that substance is in the sample. The analytical chemist has many tools at his disposal, for this quantitative experiment we will be performing a gravimetric analysis to find our how much nickel is in an unknown sample. Gravimetric analysis means to separate the nickel from the sample, and then weigh it. Your instructor will give you a test tube that contains approximately one gram of a compound that contains an unknown amount of nickel compound. You will dissolve a portion of this sample in water. When the sample dissolves, the nickel compound ionizes into Ni2+ and a negatively-charged counter ion. It does not matter what the negative ion is, because we will be separating out the nickel ion using an organic molecule called dimethyl glyoxime(DMG). When we add the DMG, it combines with the nickel ion to form a very large molecule that does not dissolve in water. The nickel DMG, or NiDMG, forms a pink solid that we can separate from solution by using a funnel and filter paper. Weighing the solid NiDMG tells us the mass, and applying the molar mass will then tell us the number of moles of NiDMG and then of nickel. It is a simple matter to find the mass of nickel from the number of moles, and finally we can compare the mass of the nickel recovered to the mass of the original unknown sample to find the percentage of nickel in the sample. The Data Analysis section of this lab has a more complete explanation of the calculations involved in this experiment.

Materials:

Equipment—beakers, Buchner funnel, filter paper, filtration apparatus, balances.

Reagents—DMG in ethanol solution, NH4OH, acetone.

Unknowns—nickel compounds.

Safety Precautions:

In addition to all regular safety precautions, be aware that nickel is a suspected carcinogen. Avoid direct contact with nickel compounds by using plastic gloves.

Experimental Procedures:

Find the mass of the unknown sample. Prepare your sample. Carefully weigh about 2.0 grams of your unknown nickel compound into a small beaker. Be precise.
Dissolve your sample in distilled water. Dissolve the sample in 20-30 mL of distilled water. You may need ...

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Materials:

Equipment—beakers, Buchner funnel, filter paper, filtration apparatus, balances.

Reagents—DMG in ethanol solution, NH4OH, acetone.

Unknowns—nickel compounds.

Safety Precautions:

In addition to all regular safety precautions, be aware that nickel is a suspected carcinogen. Avoid direct contact with nickel compounds by using plastic gloves.

Experimental Procedures:

Find the mass of the unknown sample. Prepare your sample. Carefully weigh about 2.0 grams of your unknown nickel compound into a small beaker. Be precise.
Dissolve your sample in distilled water. Dissolve the sample in 20-30 mL of distilled water. You may need to heat your beaker slightly in order to dissolve the entire sample. Be careful not to spill any of your samples.
Use a clean beaker to get about 25 mL of DMG solution.
Add NH4OH and DMG. The nickel/DMG reaction proceeds more quickly and completely if you add a small amount of NH4OH (4-5 drops) to your dissolve nickel solution before you add the DMG. Bring your beaker of dissolved nickel to the lab fume hood and add the NH4OH. The NH4OH is very concentrated so do not spill any on yourself and do not inhale any vapors.
Bring the beaker containing the nickel solution and NH4OH back to your lab bench and carefully add the DMG solution a few drops at a time. The NiDMG will come out of the solution as a pink solid. Continue adding DMG until no more NiDMG comes out of solution. Heating the solution slightly will also ensure that all of the nickel reacts with DMG. If you heat the solution, do so slowly and carefully, and only heat until your beaker is warm to touch. This solution boils over very easily. Let the beaker cool to room temperature.
Let the precipitate settle, and separate the solid and liquid with a suction filter. While your solution cools, set up your Buchner funnel apparatus. If you were to filter your precipitate using only gravity, this experiment would take days. To speed up the filtration process, you will use a vacuum filtration system. The first step you must do is weigh the clean, dry top portion of your filter apparatus, with a piece of filter paper inside. Record this weight. You will need to know the mass of the filter paper and funnel assembly for step 9. When you are ready, use the Buchner funnel to vacuum filter your NiDMG solution.
Add DMG to the liquid in your filtering flask. Add DMG to the liquid in your vacuum flask to see if more NiDMG precipitates out. Re-filter if necessary.
Dry with acetone. By now you should have a wet mass of NiDMG sitting on your filter paper. In order to obtain an accurate mass measurement, you must completely dry the NiDMG. Acetone will remove water from the NiDMG, but will not dissolve the NiDMG. Acetone also evaporates very quickly. Turn the vacuum aspirator on and use an eye dropper to add about 25 mL of acetone to the NiDMG on your filter paper; then let the NiDMG dry for five to ten minutes with the aspirator on. The precipitate will turn light pink as it dries.
Determine the mass of the precipitate on your filter paper. Weigh your filter, funnel, and precipitate. Subtract the weight of the top of the funnel and filter, and you willl have the weight of the NiDMG you have recovered.
Repeat the entire experiment two more times to show that yur results are reproducible.

Clean up. By the time you perform this experiment 3 times, you will have flasks and beakers encrusted with sticky, pink, solid NiDMG. Take time to completely clean your glassware and filtering apparatus before you leave the lab.