Inconceivable Inorganic!
A Salt ( X ) on heating in air produces a gas ( A ).
The salt, on reacting with dilute produces a gas ( B ) and a blue colored solution ( C ).
Gas ( A ), when passed through the aq. solution of ( B ), produced a yellowish white precipitate ( D ).
Salt gives a blue colored bead in oxidizing flame, and colorless bead in reducing flame, in Borax Bead test.
Find the sum of the molecular masses of A , B , C , D , and X .
The answer is 385.
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The borax bead test suggested that the metal present in the salt is copper. Under oxidative conditions, we see the blue copper(II) while under reductive conditions, we are probably observing the colourless copper(I) or copper(0). Some literature sources indicate that copper can appear red in the borax bead test under reducing conditions.
The key to solving this question is the identity of the pale yellow precipitate from the reaction of the two gases. Since this yellow solid is most likely non-metallic in nature (since it is formed by the reaction of two gases), the most likely candidate is elemental sulfur, which can be formed from the reaction between hydrogen sulfide and sulfur dioxide.
This leads us to concluding that the unknown salt is CuS.
X: CuS
A: SO2 (from heating CuS in oxygen-rich air)
B: H2S (from treating a sulfide with acid)
C: CuSO4 (there is ambiguity here since CuSO4 in solution forms the hexaaquacopper(II) complex ion)
D: S or S8 (SO2 + 2H2S -> 3S + 2H2O)
Taking all atomic masses to at least 1 decimal places, the sum of all the molecular/formula masses = 385