Question

0.99 g of organic compound containing halogen when treated with fuming nitric acid in presence of $AgN{{O}_{3}}$ in a carius tube gave 0.287 g of white precipitate. The percentage of halogen is:
a.) 29.6
b.) 7.17
c.) 35.4
d.) 64.2

Answer 1

Hint: White precipitate is forming in presence of $AgN{{O}_{3}}$ and chlorine is also present can only be the white precipitate of $AgCl$. So the halogen present over here is Chlorine. Now all we have to do is to calculate the number of moles of \[AgCl\] and that will give us the number of moles of chlorine atoms and after converting it into weight, we can calculate the percentage of chlorine.

Complete Solution :
Moles of a substance= weight of the substance/ gram molecular weight
Gram molecular weight of $AgCl$ = ${{M}_{Ag}}+{{M}_{Cl}}$
Gram molecular weight of $AgCl$ = 108+ 35.5
Gram molecular weight of $AgCl$ = 143.5g

Weight of $AgCl$ = 0.287 g
Moles of $AgCl$ = 0.287/143.5
Moles of $AgCl$ = $2\times {{10}^{-3}}$

- Now as one molecule of silver chloride contains one atom of chlorine, therefore
Number of moles of chlorine atoms = $2\times {{10}^{-3}}$
Weight of chlorine = $2\times {{10}^{-3}}\times 35.5$
Weight of chlorine = $71\times {{10}^{-3}}g$
Weight of organic compound = 0.99g
Weight of organic compound = $990\times {{10}^{-3}}g$

Percentage of chlorine = ( weight of chlorine/weight of organic compound)100%
Percentage of chlorine = $\dfrac{71\times {{10}^{-3}}}{990\times {{10}^{-3}}}\times 100%$
Percentage of chlorine = $\dfrac{71}{990}\times 100%$
Percentage of chlorine = 7.17%
So, the correct answer is “Option B”.

As halogen is chlorine that’s why white precipitate of AgCl was observed. In the case of bromine, yellow precipitate of AgBr will be observed and in case of Iodine, pale yellow precipitate of AgI will be observed.

Note: The Carius halogen method in analytical chemistry is a method for the quantitative determination of halogens in chemical substances. A known mass of an organic compound is heated with fuming nitric acid in the presence of silver nitrate contained in a hard glass tube known as carius tube, in a furnace. Carbon and hydrogen present in the compound are oxidised to carbon dioxide and water. The halogen present forms the corresponding silver halide. It is filtered, washed, dried and weighed.