Qualitative Analysis for Chemistry
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Firstly, I want to discuss with everyone some procedures carried out in this experiment involving Qualitative Analysis. A few questions is in store too.
Objective: Identify element(s) present in solid R and solution S
- the Anion present in S
Solid R contains 2 elements. The metal found in R is also present in solution S
1a Put about half of Sample R into test tube and heat strongly until no further change is observed. Next, wait for residue to cool off.
(my observation:Solid changes from brown to yellowish orange. Gas produced relights glowing splint. oxygen is released.)
2 To new sample R, add small volume of concentrated HCl and warm mixture gently and leave to stand for a few minutes.
(my observation: bubbled produced. Gas produced turns moist blue litmus paper red and is then bleached. Cl2 gas is evolved.)
3a To a portion of solution S , add NaOH until change is seen
(observation: white precipitate formed. )
3b Add NaOH in excess to mixture in (a)
observation: white ppt dissolves in excess NaOH to give colourless solution.
3c To a portion of mixture from (b), add a small piece of aluminium foil and warm gently
observation: bubbles prodiced. Red litmus paper turns blue. Ammonia gas produced and NO3- ion is present
4a To a portion of solution S, add aqueous potassium chromate(VI)
obsevation: solution S turns yellow. Yellow precipitate is produced. Hence S is reducing agent.
4b Pour small portion of mixture from (a) into boiling tube and add dilute HCl until boiling tube is almost 1/3 filled. Leave mixture to stand for a few minutes
observation : yellow precipitate turns white. Redox action has occurred.
What I find strange is that a delivery tube was given to us and not all made use of it. So I assume CO2 gas must have evolved at some point during experiment. Having said that, Qualitative Analysis experiments do not tell us which gas to test for. In this case, the gas to test for for question one is rather unclear. So how then do we know which method for testing should be applied?
And my friend said the elements present in solid R are lead and oxygen, whereas anion in solution S in NO3-. I think the answer regarding solid R's components are wrong. Can someone try to guess what it might be fromthe observations described above? They may not be entirely correct but I hope just reading through the procedure would give you a brief idea.
Please help me solve these queries of mine as I am rather eager to learn about the kind of methods for gas test I should use. I can't perform all the tests because this would take up time and maybe, the gas would have been diffused after some time.
Not forgetting, a big thanks!
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Originally posted by anpanman:
Firstly, I want to discuss with everyone some procedures carried out in this experiment involving Qualitative Analysis. A few questions is in store too.
Objective: Identify element(s) present in solid R and solution S
- the Anion present in S
Solid R contains 2 elements. The metal found in R is also present in solution S
1a Put about half of Sample R into test tube and heat strongly until no further change is observed. Next, wait for residue to cool off.
(my observation:Solid changes from brown to yellowish orange. Gas produced relights glowing splint. oxygen is released.)
2 To new sample R, add small volume of concentrated HCl and warm mixture gently and leave to stand for a few minutes.
(my observation: bubbled produced. Gas produced turns moist blue litmus paper red and is then bleached. Cl2 gas is evolved.)
3a To a portion of solution S , add NaOH until change is seen
(observation: white precipitate formed. )
3b Add NaOH in excess to mixture in (a)
observation: white ppt dissolves in excess NaOH to give colourless solution.
3c To a portion of mixture from (b), add a small piece of aluminium foil and warm gently
observation: bubbles prodiced. Red litmus paper turns blue. Ammonia gas produced and NO3- ion is present
4a To a portion of solution S, add aqueous potassium chromate(VI)
obsevation: solution S turns yellow. Yellow precipitate is produced. Hence S is reducing agent.
4b Pour small portion of mixture from (a) into boiling tube and add dilute HCl until boiling tube is almost 1/3 filled. Leave mixture to stand for a few minutes
observation : yellow precipitate turns white. Redox action has occurred.
What I find strange is that a delivery tube was given to us and not all made use of it. So I assume CO2 gas must have evolved at some point during experiment. Having said that, Qualitative Analysis experiments do not tell us which gas to test for. In this case, the gas to test for for question one is rather unclear. So how then do we know which method for testing should be applied?
And my friend said the elements present in solid R are lead and oxygen, whereas anion in solution S in NO3-. I think the answer regarding solid R's components are wrong. Can someone try to guess what it might be fromthe observations described above? They may not be entirely correct but I hope just reading through the procedure would give you a brief idea.
Please help me solve these queries of mine as I am rather eager to learn about the kind of methods for gas test I should use. I can't perform all the tests because this would take up time and maybe, the gas would have been diffused after some time.
Not forgetting, a big thanks!
This experiment may be a health hazard, and should not have been given to students.S appears to be lead(II) nitrate, which when mixed with potassium chromate(VI), produces a yellow precipitate of lead(II) chromate (which is fatally toxic and carcinogenic).
R appears to be lead(IV) oxide (stock name) or lead dioxide (prefix name), PbO2(s), a brown solid, which decomposes to oxygen gas and a yellowish-orange solid Pb3O4(s) which is actually (PbO)2.PbO2(s), a complex of lead(II)oxide and lead(IV)oxide.
Because lead(IV) oxide is a strong oxidizing agent, chloride ions are oxidized to chlorine gas.
Again, this experiment involves fatally toxic and carcinogenic materials, and should not be conducted by students.
In regard to your question on which gas to test for, you have to test for any gas you suspect is present. You'll have to pick up clues from the other tests and their results, and make educated guesses and prioritize your tests accordingly, for maximum efficiency.
Thermal decomposition of the following solids may produce the corresponding gases (note : there are exceptions) :
oxides - oxygen (rarely)
hydroxides - water vapour
carbonates - carbon dioxide
sulfates - sulfur dioxide and oxygen
nitrates - nitrogen dioxide and oxygen
Efferverscence which occurs when you add a solution to a solid, without heating, may indicate hydrogen gas (eg. metal and acid) or carbon dioxide gas (eg. carbonate and acid).
When metal halids are heated with concentrated sulfuric acid (usually too hazardous for students to handle), steamy fumes of hydrogen halides and halogen gases may be produced.
We Love Qualitative Analysis! -
No!
I hate qualitative analysis, much less quantitative analysis!
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Originally posted by UltimaOnline:
This experiment may be a health hazard, and should not have been given to students.S appears to be lead(II) nitrate, which when mixed with potassium chromate(VI), produces a yellow precipitate of lead(II) chromate (which is fatally toxic and carcinogenic).
R appears to be lead(IV) oxide (stock name) or lead dioxide (prefix name), PbO2(s), a brown solid, which decomposes to oxygen gas and a yellowish-orange solid Pb3O4(s) which is actually (PbO)2.PbO2(s), a complex of lead(II)oxide and lead(IV)oxide.
Because lead(IV) oxide is a strong oxidizing agent, chloride ions are oxidized to chlorine gas.
Again, this experiment involves fatally toxic and carcinogenic materials, and should not be conducted by students.
In regard to your question on which gas to test for, you have to test for any gas you suspect is present. You'll have to pick up clues from the other tests and their results, and make educated guesses and prioritize your tests accordingly, for maximum efficiency.
Thermal decomposition of the following solids may produce the corresponding gases (note : there are exceptions) :
oxides - oxygen (rarely)
hydroxides - water vapour
carbonates - carbon dioxide
sulfates - sulfur dioxide and oxygen
nitrates - nitrogen dioxide and oxygen
Efferverscence which occurs when you add a solution to a solid, without heating, may indicate hydrogen gas (eg. metal and acid) or carbon dioxide gas (eg. carbonate and acid).
When metal halids are heated with concentrated sulfuric acid (usually too hazardous for students to handle), steamy fumes of hydrogen halides and halogen gases may be produced.
We Love Qualitative Analysis!
Your explanation helps a lot. May I know in which part of the experiment indicate that Pb2+ ion was present in solution S and solid R? -
Originally posted by anpanman:
Your explanation helps a lot. May I know in which part of the experiment indicate that Pb2+ ion was present in solution S and solid R? -
Originally posted by anpanman:
Your explanation helps a lot. May I know in which part of the experiment indicate that Pb2+ ion was present in solution S and solid R?
The cation in question gave a white ppt with NaOH(aq), which dissolved in excess NaOH(aq). Hence, the cation could be either Al3+, Pb2+, or Zn2+.Of these 3 cations, only Pb2+ qualifies as a cation of a transition metal* (that has partially filled d-orbitals), and hence only Pb2+ forms coloured compounds (eg. the yellow lead(II) chromate ppt).
(*Note : although zinc is a d-block metal, it is not a transition metal. It does not form coloured compounds.)
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I am surprised if MOE allows lead compounds to be used for the experimental section.
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Ah, classical wet analytical chemistry. While it is quite dated and does not represent modern analytical chemistry, the material is by no means irrelevant.
That's where I got my foundation (training) in good laboratory habits from, and the attention paid to record details accurately would pay off for anyone who is considering a career in the chemical and allied sciences.
Now of course, modern techniques such as NMR, IR spectroscopy, MS etc are routinely used in the labs instead for chemical identification.