Question

when concentrated sulphuric acid was added to an unknown salt present in a test tube a brown gas (a) was evolved. this gas intensified when copper turnings were added to this test tube. on cooling, the gas (a) changed into a colourless solid (b) (i) identify (a) and (b) (ii) write the structures of (a) and (b) (iii) when does gas (a) change to solid on cooling?

Answer 1

A is pb(no3)2 B is n2o4 pb(no3)2-->2pbo+ 4no2+o2 2no2--- cooling---> n2o4(colourless)

Answer 2

(i) Identification of (a) and (b)

• The gas which is brown in color is nitrogen dioxide gas. Thus gas (a) is identified as Nitrogen dioxide gas.
• When the gas is cooled, it gets oxidized to solid substance called Dinitrogen Tetraoxide and its molecular formula is $N_2O_4$.
• Thus (b) is identified as Dinitrogen Tetraoxide.

(ii) Structures of (a) and (b)

• The structure of gas (a) called nitrogen gas and solid (b) called Di-nitrogen Tetraoxide is given in the attachment.

(iii) Reason for conversion of gas to solid on cooling

• As nitrogen dioxide contains odd number of electrons, it readily looses them.
• Due to this reason, it gets converted into solid Dinitrogen Tetraoxide on cooling.

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