Analysis of two unknown inorganic compounds

 

Question

C and D are samples of two different simple salts. Carry out the following exercises on them. Record your observations and identify any gases evolved. State the conclusion you draw from the result of each test.

A)    Heat about one half of C in dry test tube until no further change is observed. Allow to cool

B)     (I)  to the cooled residue from (a) above, add about 5 cm³ of dilute hydrochloric acid and warm.

(II)  to about 2cm³ of the clear solution from (b)(I) above, add aqueous ammonia in drops until it is excess.

C)    Put all D in a boiling tube and add about 10 cm³ of distilled water. Shake thoroughly and divide into two portions.

D)    (I) to the first portion from (C) above, add about 2 cm³ of barium chloride solution, followed by dilute hydrochloric acid in excess. Warm the mixture.

(II) to the second portion from ( C) above, add 2 to 3 drops  of acidified  potassium tetraoxomanganate (VII) solution and shake. (_{The question is credit to West Africa Senior Secondary Certificate Examination)}

 Answer

C is Copper trioxocarbonate (IV) and D is sodium trioxosulphate (IV)

 

S\n	Test	Observation	Inference
A (I)	C  + heat	Colourless ,odourless gas which is acidic to litmus paper and turned lime water milky	CO2, present, CO32-,HCO3- present
(II)	On cooling	green colour turned to black	Oxide of Cu, sulphide of Cu, Pb and carbon likely present
B(I)	Black residue + dil.HCl + heat.	No gas evolved,  dissolved to give a blue solution	Sulphide absent, carbon absent, soluble chloride present
(II)	Solution from (b)(I) +aqueous ammonia in drops till excess	Pale blue precipitate in drops but soluble in excess to form a deep blue solution	Cu2+ confirmed
C(I)	D + water divide into two portions	Dissolved and formed a colourless solution	Soluble salts Na+,K+,NH4+ likely present
D(I)	To the first portion of solution from (C) above + dil. Barium chloride + dil. HCl +heat	White precipitate formed which dissolved in dil. HCl and produced a colourless gas with irritating smell	SO42- absent, CO32-     and HCO3- absent, SO32- and SO2 likely present
(II)	To the second portion add acidified KMnO4 solution and shake	It decolourized KMnO4 ,purple colour turned colourless)	SO2, SO32- confirmed

 

Explanation of the result\inference

A.    In test (a)(I) C liberated a gas which turned lime water milky. this  means that carbon (IV)oxide is present and this gas can only come from Trioxocarbonates or hydrogen carbonates

B.      In test (a)(II)the black residue can be one of the following compounds, CuO, Cus, PbS, or carbon which are common black compounds.

C.    In test (b)(I) the black residue dissolved in warm dil. HCl without giving off gas suggested that sulphide, and lead ions are likely to be absent. This is because lead has insoluble chloride while sulphide liberates hydrogen sulphide gas when heated with dil. HCl.

D.     In test (b)(II) the solution from b(I) formed pale blue precipitate which was soluble in excess aqueous ammonia to form deep blue solution. This shows that copper ions are present.

E.      In test (c)(I)D dissolved in water and it gave colourless solution. This means that insoluble salts are not present. The possible ions might be sodium ions, potassium ions and ammonia ions. This is because they are always soluble in every case.

F.       In test (d)(I) the solution plus barium chloride and dil. Hydrochloric acid plus heating. The white precipitate formed became soluble and produced a gas which has an irritating smell. This means that SO₄^2- and carbonate are absent. Trioxosulphate (IV) and SO₂ likely present.

G.    In test (d)(II) the solution decolorized KMnO₄. This means that SO₂ and trioxosulphate (IV) are present. Note hydrogen sulphide can do the same but with yellow deposit of sulphur.