Inorganic Qualitative Analysis - Questions and Answers

To identify compounds, we have to follow some steps in chemistry. Qualitative analysis teaches us how to identify one compound from other compound.

Example: How to identify dilute HCl acid from dilute H2SO4 acid in the laboratory?



We do our testing for anions and cations. Different cations show different characteristics. Also anions are also show like that. In this tutorial, we will learn, these things in detail. At the end of the tutorial you should try to explain how a compound is identified from another compound by following what you learnt in qualitative analysis.



Qualitative analysis Questions for compounds which have same cation and different anions

In this section, we are going to identify anions when cation is common in other compounds.



Question 1.1

How to identify Na2S and Na2SO4 aqueous solutions?


Answer

Both of Na2S and Na2SO4 are solutions and also colourless. Therefore we cannot identify it by colour. Sodium ion is common for both compounds. So we have to test the anions sulfide ion S2- and sulfate ion (SO42-).


Testing sulfide ion S2- and sulfate ion (SO42-)

Add BaCl2 to both solutions separately. We can see a white precipitate forms in a one solution (Na2SO4 solution). That precipitate is BaSO4. But BaS is soluble in water and colourless too.

identify sodium sulfide and sodium sulfate



Question 1.2

Give a method to identify NaBr and NaCl which are in solid state.


Answer

NaCl and NaBr are white solids. Sodium ion is common in both NaCl and NaBr. Also both NaCl and NaBr are soluble in water and cannot see a precipitate to identify a compound.


We use silver nitrate solution to identify halide ions.

With AgNO3, NaCl and NaBr gives AgCl and AgBr precipitates respectively.

AgCl is a white color insoluble precipitate and AgBr is a pale yellow color insoluble precipitate.



Questions of identify cations in compounds - qualitative analysis

Here, we are going to study about, how to identify a cation from another cation when anion is common in those compounds.



Question 2.1

How to identify aqueous NaCl and KCl solutions?


Answer

Both sodium and potassium are alkali metals. NaCl and KCl are colorless aqueous solutions. So we cannot see a color change in two compounds and unable to identify NaCl from KCl from colors of solution.

Flame tests to identify Na+ and K+ ions

Na+ and K+ give yellow and purple colors respectively for flame test.



Question 2.2

In qualitative analysis questions, remembering colors of compounds is required. From following set, which compound's color is wrong?

  1. Aqueous CuSO4 is blue
  2. Anhydrous sodium carbonate is brown.
  3. Aqueous ferric chloride is brown colour.
  4. Both PbCl2 and AgCl are white precipitates.

Answer

Answer is 2. Anhydrous sodium carbonate is white.



Question 2.2

M is a cation which exists as a nitrate compound in an aqueous solution. Part of this solution is separated and H2S gas is sent. Then a black precipitate is given. Next, another part of this M's nitrate solution is separated. When NaOH is added, white precipitate is given. With excess NaOH, white precipitate dissolve and give colourless solution. What could be the cation?

  1. Ag+
  2. Na+
  3. Fe2+
  4. Al3+
  5. Pb2+

Answer

Pb2+

PbS is a black precipitate. See all inorganic metal sulfide precipitates and solubility.

Pb(OH)2 is a white precipitate. See all inorganic metal hydroxide precipitates and solubility.





Lewis structures

P2O5 lewis structure OH- lewis structure Ammonium ion (NH4+) lewis structure H2CO3 lewis structure