Analytical Chemistry Question 197
Question: The Lassaigne ’s extract is boiled with dil. $ HNO_3 $ before testing for halogens because
Options:
A) silver halides are soluble in $ HNO_3 $
B) $ Na_2S $ and NaCN are decomposed by $ HNO_3 $
C) $ Ag_2S $ is soluble in $ HNO_3 $
D) $ AgCN $ is soluble in $ HNO_3 $
Show Answer
Answer:
Correct Answer: B
Solution:
[b] $ Na_2S $ and $ NaCN, $ formed during fusion with metallic sodium, must be removed before adding $ AgNO_3 $ , otherwise black ppt. due to $ Na_2S $ or white precipitate due to $ AgCN $ will be formed and thus white precipitate of $ AgCl $ will not be identified easily. $ Na_2S+2AgNO_3\xrightarrow{{}}2NaNO_3+\underset{Black}{\mathop{Ag_2S}}\downarrow $
$ NaCN+AgNO_3\xrightarrow{{}}NaNO_3+\underset{White}{\mathop{AgCN}}\downarrow $
$ NaCl+AgNO_3\xrightarrow{{}}NaNO_3+\underset{White}{\mathop{AgCl}}\downarrow $
$ Na_2S+2HNO_3\xrightarrow{boil}2NaNO_3+H_2S\uparrow $
$ NaCN+HNO_3\xrightarrow{boil}NaNO_3+HCN\uparrow $
BETA
