Read the passage carefully and answer the questions.
The coordination compounds are of great importance. These compounds are widely present in the mineral, plants and animal world and are known to play many important functions in the area of analytical chemistry, metallurgy, biological systems, industry and medicine. For example, coordination compounds find use in many qualitative and quantitative chemical analysis. Some important metal extraction and purification make use of complex formation. Hardness of water can be estimated by stable complex formation of \( \text{Ca}^{2+} \) and \( \text{Mg}^{2+} \)
In photography undecomposed AgBr is removed as ………
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → \( [\text{Ag}(\text{S}_2\text{O}_3)_2]^{3-} \)
In black and white photography, the silver bromide that was struck by light is reduced to black metallic silver when the film is developed. The silver bromide that was not exposed must then be removed; otherwise it would darken later in daylight and spoil the picture. This step is called fixing.
The film is washed with hypo, sodium thiosulphate. Insoluble AgBr dissolves because the thiosulphate ion forms a soluble complex with \( \ce{Ag+} \), sodium dithiosulphatoargentate(I):
\( \displaystyle \ce{AgBr + 2Na2S2O3 -> Na3[Ag(S2O3)2] + NaBr} \)
The thiosulphate ion carries a charge of −2, so the oxidation state of silver is:
\( \displaystyle x + 2(-2) = -3 \Rightarrow x = +1 \)
So the undecomposed AgBr is removed as \( \ce{[Ag(S2O3)2]^3-} \).
NCERT (2026-27 edition):
Importance and Applications of Coordination Compounds – …
