Question

An organic compound contains `C,H,N,S` and Cl. For the detection of chlorine the sodium extract of the compound is first heated with a few dropsof concentrated `HNO_(5)` and then `AgNO_(3)` is added to get a white ppt of `AgCl`. The digestion with `HNO_(3)` before the addition of `AgNO_(3)` is
A. To prevent the formation of `AgNO_(3)` is
B. To create a common ion effect
C. To convert `CN^(-)` and `S^(2-)` to volatile `HCN`and `H_(2)S`, or else they will interfere with the test forming `AgCN` or `Ag_(2)S`.
D. To prevent the hydrolysis of `NaCN` and `Na_(2)S`

Answer 1

Correct Answer - C
`Na+C+Noverset(Delta)rarrNaCN, 2Na+S overset(Delta)rarr Na_(2)S`
Both `NaCN` and `Na_(2)S` will react with `AgNO_(3)` to form
`AgCN` (white ppt.) and `Ag_(2)S` (black pt.)
They must be removed before performing `AgNO_(3)` test for halogens Thus, on adding `HNO_(3),NaCN` and `Na_(2)S`
`NaCN + HNO_(3) rarr NaNO_(3) + HCN uarr`
`Na_(2)S + 2HNO_(3) rarr 2 NaNO_(3) + H_(2)S uarr`
Hence, `CN^(-)` and `S^(2-)` are removed as HCN and `H_(2)S` by adding `HNO_(3)`. Hence, choice (c) is correct while (a), (b) and (c) incorrect.