Qualitative analysis lab idea A Chapter 2 lab Issues: too many

Report
Name:
Qualitative Analysis
Laboratory
30 points
For your laboratory report, please turn in just
answers to the questions on the last slide.
Handwritten responses will not be accepted.
Introduction
Colorimetric methods of analysis have been developed for most
of the elements and ionic species. These methods can be
employed to quickly determine the chemical composition of an
unknown substance or solution. Recall that in solution, ionic
compounds can either separate into free ions (soluble) or form
precipitates (insoluble salts).
Before attempting to analyze an unknown it is useful to develop
a logical process to follow. Care must be taken in sampling,
collecting and washing precipitates, performing confirmatory
tests and most importantly, recording data. It is also good
laboratory practice to always analyze a known with each
unknown as a double check on the reagents being used.
Introduction (cont’d)
The halides (chlorine, bromine and iodine) all form insoluble
salts (precipitates) with silver. Silver chloride is a white
precipitate where as silver bromide is beige and silver iodide is
yellow. The presence of silver chloride can be confirmed by
dissolving the precipitate in ammonium hydroxide. Other silver
halides are not soluble in ammonium hydroxide. Bromine can
be confirmed using potassium permanganate and iodine can be
confirmed with ferric nitrate reagent.
Phosphate ions can be detected using ammonium molybdate
reagent and stannous chloride. Nitrates can be detected using
diphenylamine reagent, and sulfate using barium chloride
reagent.
Objective
You will be provided with three unknown
solutions containing any combination of six
possible anions:
ClBrIPO42NO3SO42Determine which ions are present in each of the
three unknown solutions.
Materials
Test Solutions
100mL 0.1M NaBr
100mL 0.1M NaCl
100mL 0.1M NaI
100mL 0.1M NaNO3
100mL 0.1M Na3PO4
100mL 0.1M Na2SO4
Equipment
•disposable pipets
•test tubes / rack
•stoppers
•funnels
•filter paper
•scupula
•deionized water
•waste container(s)
Safety: the nitric acid and ammonium
hydroxide can cause severe burns and
shortness of breath if inhaled. Wear
goggles and wash you hands thoroughly
after handling the chemicals!
Other Solutions
•3 Unknown Solutions
•225mL 6M HNO3
•100mL 6M NH4OH
•100mL mineral oil
•25mL (NH4)6Mo7O24
(ammonium molybdate)
•25mL SnCl2
•25mL (C6H5)2NH
(diphenylamine reagent)
•25mL BaCl2
•25mL AgNO3
•25mL KMnO4
•25mL Fe(NO3)3
Procedure
Halides
1) Label one test tube for each test and unknown solution . Using a
disposable pipet, add about 1mL (about 20 drops) of the 3 test solutions and
the 3 unknown solutions to the appropriate test tubes. Add a few drops of
acid to each tube.
2) Add about 10 drops of AgNO3reagent to each tube and mix. A precipitate
(solid) will form in any tube containing halides.
3) Using filter paper and a funnel, filter the liquid from those tubes that have
a precipitate and wash the precipitate with deionized water. Transfer (scrape)
precipitates into new test tubes.
4) Add 0.5 ml (about 10 drops) 6M NH4OH to each precipitate.
5) Into new test tubes, place fresh 1.0 mL (20 drop) samples of the 3 test
solutions and any unknown solutions that tested positive for halides. Acidify
the samples as in step 1.
6) Add 1.0 mL Fe(NO3)3 reagent and mix. Add 1.0 mL of mineral oil, stopper
the tube and shake to mix. Allow the layers to separate. A pink/purple color
in the oil level confirms the presence of iodide.
7) Pour off the mineral oil layer and discard. Add 1.0mL of KMnO4 reagent to
the tube and mix. Add 1.0mL of mineral oil, stopper the tube and shake to
mix. Allow the layers to separate. A yellow to brown color in the oil level
confirms the presence of bromine.
Observations: Halide Test
added
Unknown
1
Unknown
2
Unknown
3
NaCl
NaI
NaBr
white ppt
no ppt
beige ppt
white ppt
yellow ppt
beige ppt
---
ppt
insoluble
ppt
dissolved
ppt
insoluble
ppt
insoluble
---
pink oil
layer
---
---
yellow oil
layer
AgNO3
NH4OH
Fe(NO3)3 +
mineral oil
KMnO4 +
mineral oil
ppt
dissolved
---
---
---
---
---
yellow oil
layer
---
Procedure (cont’d)
Phosphate
8) Add about 1mL (20 drops) of the Na3PO4 test solution and 3 unknown
solutions to labeled test tubes.
9) Add 1.0mL of the ammonium molybdate reagent to the test tubes and
mix. Add one to two drops of SnCl2 reagent and mix. The formation of a blue
color is a positive test for phosphate. A green or yellow color is negative.
Nitrate
10) Label one test tube for the NaNO3 test solution and the 3 unknown
solutions. Add about 1mL of the test and unknown solutions to the
appropriate test tubes.
11) Add 1.0mL of the diphenylamine reagent to the test tubes and mix. The
formation of a blue color is a positive test for nitrate.
Sulfate
12) Label one test tube for the Na2SO4 test solution and the 3 unknown
solutions. Add 1mL of the test and unknown solutions to the appropriate test
tubes.
13) Add a few drops of nitric acid to each tube and mix.
14) Add 1.0mL of BaCl2 solution to each tube and mix. The formation of a
white precipitate is a positive test for sulfate.
Observations: Polyatomic Ions Test
added
Unknown
1
Unknown
2
Unknown
3
Na3PO4
NaNO3
Na2SO4
green/
yellow
green
green
green
green
green/
yellow
(C6H5)2NH
clear
clear
blue
clear
blue
clear
HNO3 + BaCl2
white ppt
no ppt
no ppt
no ppt
no ppt
white ppt
(NH4)6Mo7O24
+ SnCl2
Laboratory Report Guidelines
Data Analysis
1) (5pts) Explain why Cl-, Br-, I- are all called ‘halides.’
2) (5pts) Give the chemical names for each of the unnamed
chemicals (see Materials section).
3) (5pts) When you tested for the halides, you used silver
nitrate to check for precipitation. Write the equations
(complete or net ionic) showing the formation of these
precipitates.
Conclusion
1) (5pts) Which anions are present in which unknown
solutions? Explain the details behind your reasoning.
2) (5pts) How does this experiment apply to the real world?
Give two specific examples.
Error Analysis
1) (5pts) What was the purpose of the 6 test solutions if we
already knew what ions they contained?

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