Question

What happens when-
Two gases ‘A’ and ‘B’ turn acidified ${K_2}C{r_2}{O_7}$ green. When these gases are passed through water simultaneously the solution turns turbid yellow. What are the gases ‘A’ and ‘B’?

Answer 1

Hint: Gas A is weakly acidic and also behaves as a Lewis base while gas B behaves as a reducing agent which reduces the oxidation state of chromium making the solution orange to green. The yellowish turbidity is produced due to sulphur. Both the gases contain sulphur in its composition.

Complete answer:
Potassium dichromate, ${K_2}C{r_2}{O_7}$ , is a typical inorganic chemical reagent that is most commonly employed as an oxidising agent in lab and industrial settings. It is immediately and chronically detrimental to health, as are all hexavalent chromium compounds. It has a vivid red-orange colour and is a crystalline ionic solid. In contrast to the more industrially significant salt sodium dichromate, the salt is popular in the laboratory because it is not deliquescent.
The given information suggests that the gas ‘A’ is ${H_2}S$ and the gas ‘B’ is $S{O_2}$ . Both turn acidified ${K_2}C{r_2}{O_7}$ paper green.
For gas ‘A’:-
${K_2}C{r_2}{O_7} + 4{H_2}S{O_4} + 3{H_2}S \to {K_2}S{O_4} + C{r_2}{(S{O_4})_3}(green) + 7{H_2}O + 3(s)$
For gas ‘B’:- (Sulphur dioxide test)
${K_2}C{r_2}{O_7} + {H_2}S{O_4} + 3S{O_2}(gasB) \to {K_2}S{O_4} + C{r_2}{(S{O_4})_3}(green) + {H_2}O$
For both gas ‘A’ and gas ‘B’:-
$
  2{H_2}S + S{O_2} \to 2{H_2}O + 3s \\
  (gasA)\,\,\,(gasB)\,\,\,\,\,\,\,\,\,(yellowish\,turbidity) \\
 $
So, gas ‘A’ is ${H_2}S$ and gas ‘B’ is $S{O_2}$ .

Note:
Potassium dichromate is one of the most common causes of chromium dermatitis; chromium is known to produce hypersensitivity and dermatitis, particularly in the hands and forearms, which can be chronic and difficult to cure. Toxicological research has added to the evidence of its severe toxicity. Concentrations as low as \[14{\text{ }}mg.k{g^{ - 1}}\] have resulted in a $50\% $ death rate across test groups in rabbits and rats. If aquatic organisms are exposed, they are especially vulnerable, hence proper disposal in accordance with local environmental standards is recommended.