The coordination compounds are of great importance. These compounds are widely present in the mineral, plant and animal worlds and are known to play many important functions in the area of analytical chemistry, metallurgy, biological systems, industry and medicine.
-- NCERT Class 12 Chemistry, Ch. 5, p. 136Applications Coordination
8 MCQs9-step worked example
Source: NCERT Coordination CompoundsOfficial key: NTA-verifiedLast updated: 8 Oct 2026
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The hardness of water is estimated by simple titration with
- A.Na2EDTA
- B.dimethylglyoxime (DMG)
- C.cupron
- D.hypo
Tap to see the answer
Answer: A. Hardness of water is estimated by titration with Na2EDTA, since Ca²⁺ and Mg²⁺ form stable complexes with EDTA (NCERT Class 12 Chemistry, Chapter 5, page 136).
B is wrong: B is wrong because DMG is listed among the reagents for detecting and estimating metal ions by colour reactions, not for water hardness (NCERT Class 12 Chemistry, Chapter 5, page 136).
C is wrong: C is wrong because cupron is another analytical reagent from the same list, not the titrant for hardness (NCERT Class 12 Chemistry, Chapter 5, page 136).
D is wrong: D is wrong because hypo is used in photography to dissolve undecomposed AgBr (NCERT Class 12 Chemistry, Chapter 5, page 137).
Applications Coordination, explained for NEET
Start with a question: why would anyone dissolve gold in a cyanide solution, measure water hardness with a compound called EDTA, and treat lead poisoning with the same EDTA? The answer in each case is a coordination compound: a metal ion held by ligands so firmly that its behaviour changes. This topic is about what those complexes are used for.
Coordination compounds are of great importance. They are widely present in the mineral, plant and animal worlds and play many functions in analytical chemistry, metallurgy, biological systems, industry and medicine (NCERT Class 12 Chemistry, Chapter 5, page 136).
Analysis. Colour reactions between metal ions and ligands, especially chelating ligands, form the basis of detection and estimation. Examples of such reagents include EDTA, DMG (dimethylglyoxime), α-nitroso-β-naphthol and cupron (NCERT Class 12 Chemistry, Chapter 5, page 136). Hardness of water is estimated by titration with Na2EDTA. Ca²⁺ and Mg²⁺ form stable complexes with EDTA, and the two can be estimated selectively because their stability constants differ (NCERT Class 12 Chemistry, Chapter 5, page 136).
Metallurgy. Gold combines with cyanide in the presence of oxygen and water to form [Au(CN)2]⁻ in solution, and metallic gold is separated from it by adding zinc (NCERT Class 12 Chemistry, Chapter 5, page 136). Metals can also be purified by forming a coordination compound and then decomposing it: impure nickel is converted to [Ni(CO)4], which decomposes to yield pure nickel (NCERT Class 12 Chemistry, Chapter 5, pages 136 and 137).
Biology. Chlorophyll is a coordination compound of magnesium, haemoglobin (the oxygen carrier) of iron, and vitamin B12 (cyanocobalamine) of cobalt. Enzymes such as carboxypeptidase A and carbonic anhydrase also contain coordinated metal ions (NCERT Class 12 Chemistry, Chapter 5, page 137).
Industry and medicine. The Wilkinson catalyst, [(Ph3P)3RhCl], is a rhodium complex used for the hydrogenation of alkenes. Silver and gold electroplate more smoothly from [Ag(CN)2]⁻ and [Au(CN)2]⁻ than from simple metal ions. In black and white photography, hypo dissolves undecomposed AgBr to form [Ag(S2O3)2]³⁻. In chelate therapy, D-penicillamine and desferrioxime B remove excess copper and iron, EDTA treats lead poisoning, and cis-platin and related platinum compounds inhibit tumour growth (NCERT Class 12 Chemistry, Chapter 5, page 137).
Watch out: pair each metal with its job. Mg goes with chlorophyll, Fe with haemoglobin, Co with vitamin B12, Pt with cis-platin, Rh with the Wilkinson catalyst.
How do you solve a Applications Coordination question? A worked example
- 1
Given
In the extraction of gold, the metal ends up in solution as the complex ion [Au(CN)2]⁻. Zinc is then added.
- 2
Required
The oxidation state of gold in [Au(CN)2]⁻, and the change in its oxidation state when zinc recovers the metal.
- 3
Concept
Gold combines with cyanide in the presence of oxygen and water to form [Au(CN)2]⁻, and metallic gold is separated from the solution by adding zinc (NCERT Class 12 Chemistry, Chapter 5, page 136).
- 4
Formula
Charge on the complex = oxidation state of gold + sum of the ligand charges.
- 5
Substitution
The charge is −1 and there are two CN⁻ ligands, each −1, so −1 = x + 2(−1).
- 6
Calculation
x = −1 + 2 = +1. The charges are exact integers, so there is no significant-figure limit.
- 7
Final answer
Gold is +1 in [Au(CN)2]⁻. Recovered as the metal, its oxidation state is 0, so gold is reduced from +1 to 0 while zinc supplies the electrons.
- 8
Common trap
Writing −1 = x + 2(+1) gives x = −3. The ligand CN⁻ carries a negative charge, so it lowers the sum, not raises it.
- 9
Similar NEET-style question
What is the oxidation state of silver in [Ag(S2O3)2]³⁻, the ion formed when hypo dissolves AgBr? (Answer: −3 = x + 2(−2), so x = +1.)
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Can you answer these Applications Coordination MCQs?
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
MCQ 1Easy RecallPractice
The hardness of water is estimated by simple titration with
Show answer and why every option is right or wrong
Answer: A. Hardness of water is estimated by titration with Na2EDTA, since Ca²⁺ and Mg²⁺ form stable complexes with EDTA (NCERT Class 12 Chemistry, Chapter 5, page 136).
Why B is wrong: B is wrong because DMG is listed among the reagents for detecting and estimating metal ions by colour reactions, not for water hardness (NCERT Class 12 Chemistry, Chapter 5, page 136).
Why C is wrong: C is wrong because cupron is another analytical reagent from the same list, not the titrant for hardness (NCERT Class 12 Chemistry, Chapter 5, page 136).
Why D is wrong: D is wrong because hypo is used in photography to dissolve undecomposed AgBr (NCERT Class 12 Chemistry, Chapter 5, page 137).
MCQ 2Easy RecallPractice
Haemoglobin, the red pigment of blood that carries oxygen, is a coordination compound of
Show answer and why every option is right or wrong
Answer: B. Haemoglobin is a coordination compound of iron (NCERT Class 12 Chemistry, Chapter 5, page 137).
Why A is wrong: A is wrong because magnesium is the metal in chlorophyll, the pigment of photosynthesis (NCERT Class 12 Chemistry, Chapter 5, page 137).
Why C is wrong: C is wrong because cobalt is the metal in vitamin B12, cyanocobalamine (NCERT Class 12 Chemistry, Chapter 5, page 137).
Why D is wrong: D is wrong because platinum appears in cis-platin, which inhibits tumour growth, not in blood pigment (NCERT Class 12 Chemistry, Chapter 5, page 137).
MCQ 3Easy RecallPractice
The Wilkinson catalyst, used for the hydrogenation of alkenes, is
Show answer and why every option is right or wrong
Answer: A. The Wilkinson catalyst is the rhodium complex [(Ph3P)3RhCl], used for the hydrogenation of alkenes (NCERT Class 12 Chemistry, Chapter 5, page 137).
Why B is wrong: B is wrong because [Ni(CO)4] is the intermediate in the purification of nickel, not a hydrogenation catalyst (NCERT Class 12 Chemistry, Chapter 5, page 137).
Why C is wrong: C is wrong because [Ag(CN)2]⁻ is the species used for smooth electroplating of silver (NCERT Class 12 Chemistry, Chapter 5, page 137).
Why D is wrong: D is wrong because [Au(CN)2]⁻ is the species formed when gold is extracted and also used for electroplating gold (NCERT Class 12 Chemistry, Chapter 5, page 136).
MCQ 4Direct ApplicationPractice
In the cyanide process, gold dissolves as [Au(CN)2]⁻ in the presence of oxygen and water. Metallic gold is recovered from this solution by adding
Show answer and why every option is right or wrong
Answer: A. Gold is separated in metallic form from the [Au(CN)2]⁻ solution by the addition of zinc (NCERT Class 12 Chemistry, Chapter 5, page 136).
Why B is wrong: B is wrong because Na2EDTA is the titrant for estimating water hardness, not a reducing agent for gold (NCERT Class 12 Chemistry, Chapter 5, page 136).
Why C is wrong: C is wrong because hypo dissolves AgBr in photography and does not precipitate gold (NCERT Class 12 Chemistry, Chapter 5, page 137).
Why D is wrong: D is wrong because more cyanide only keeps gold in solution as the complex; the metal comes out on adding zinc (NCERT Class 12 Chemistry, Chapter 5, page 136).
MCQ 5Direct ApplicationPractice
After development, a black and white photographic film is washed with hypo solution. The undecomposed AgBr is removed as
Show answer and why every option is right or wrong
Answer: A. Hypo dissolves the undecomposed AgBr by forming the complex ion [Ag(S2O3)2]³⁻ (NCERT Class 12 Chemistry, Chapter 5, page 137).
Why B is wrong: B is wrong because [Ag(CN)2]⁻ is the ion used for electroplating silver, and no cyanide is involved in fixing a film (NCERT Class 12 Chemistry, Chapter 5, page 137).
Why C is wrong: C is wrong because the page says hypo dissolves AgBr to form a complex ion, which stays in solution rather than precipitating (NCERT Class 12 Chemistry, Chapter 5, page 137).
Why D is wrong: D is wrong because hypo dissolves undecomposed AgBr; it does not reduce it to the metal (NCERT Class 12 Chemistry, Chapter 5, page 137).
MCQ 6Direct ApplicationPractice
Which sequence correctly describes the purification of impure nickel through a coordination compound?
Show answer and why every option is right or wrong
Answer: A. Impure nickel is converted to [Ni(CO)4], which is decomposed to yield pure nickel; this is the purification of a metal through formation and decomposition of its coordination compound (NCERT Class 12 Chemistry, Chapter 5, page 137).
Why B is wrong: B is wrong because the zinc step belongs to the extraction of gold from [Au(CN)2]⁻, not to nickel purification (NCERT Class 12 Chemistry, Chapter 5, page 136).
Why C is wrong: C is wrong because it forms [Ni(CO)4] from a nitrate, whereas the page converts impure nickel itself into [Ni(CO)4] and then decomposes it (NCERT Class 12 Chemistry, Chapter 5, page 137).
Why D is wrong: D is wrong because EDTA titration is an analytical method for estimating ions, not a purification route (NCERT Class 12 Chemistry, Chapter 5, page 136).
MCQ 7CalculationPractice
Match each condition with the chelating agent or complex used against it. Conditions: (i) lead poisoning, (ii) excess copper, (iii) growth of tumours. Agents: (a) EDTA, (b) D-penicillamine, (c) cis-platin. Which set of pairs is correct?
Show answer and why every option is right or wrong
Answer: A. EDTA is used in the treatment of lead poisoning, excess copper and iron are removed by D-penicillamine and desferrioxime B, and cis-platin and related platinum compounds inhibit the growth of tumours (NCERT Class 12 Chemistry, Chapter 5, page 137).
Why B is wrong: B is wrong because it swaps (i) and (ii); lead poisoning is treated with EDTA and excess copper with D-penicillamine (NCERT Class 12 Chemistry, Chapter 5, page 137).
Why C is wrong: C is wrong because it swaps (ii) and (iii); cis-platin acts on tumours and D-penicillamine removes excess copper (NCERT Class 12 Chemistry, Chapter 5, page 137).
Why D is wrong: D is wrong because it puts cis-platin on lead poisoning and EDTA on tumours, reversing (i) and (iii) (NCERT Class 12 Chemistry, Chapter 5, page 137).
MCQ 8CalculationPractice
Consider these statements. 1: Gold dissolved as [Au(CN)2]⁻ is recovered by adding zinc. 2: Ca²⁺ and Mg²⁺ can be estimated selectively with EDTA because their EDTA complexes have different stability constants. 3: Chlorophyll is a coordination compound of iron. 4: The Wilkinson catalyst is used for the dehydrogenation of alkenes. Which are correct?
Show answer and why every option is right or wrong
Answer: A. Statements 1 and 2 are both stated: zinc recovers gold from [Au(CN)2]⁻, and the difference in stability constants of the calcium and magnesium complexes allows selective estimation. Chlorophyll contains magnesium and the Wilkinson catalyst hydrogenates alkenes (NCERT Class 12 Chemistry, Chapter 5, pages 136 and 137).
Why B is wrong: B is wrong because statement 3 is false; chlorophyll is a coordination compound of magnesium, while iron is in haemoglobin (NCERT Class 12 Chemistry, Chapter 5, page 137).
Why C is wrong: C is wrong because statement 4 is false; the Wilkinson catalyst is used for hydrogenation, not dehydrogenation (NCERT Class 12 Chemistry, Chapter 5, page 137).
Why D is wrong: D is wrong because both 3 and 4 are false: chlorophyll holds magnesium and the Wilkinson catalyst hydrogenates alkenes (NCERT Class 12 Chemistry, Chapter 5, page 137).
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What to remember before solving Applications Coordination questions
11 NCERT lines
Key Fact
Qualitative and quantitative analysis: colour reactions with chelating ligands (EDTA, DMG, cupron)
Coordination compounds find use in many qualitative and quantitative chemical analysis. The familiar colour reactions given by metal ions with a number of ligands (especially chelating ligands), as a result of formation of coordination entities, form the basis for their detection and estimation by classical and instrumental methods of analysis. Examples of such reagents include EDTA, DMG (dimethylglyoxime), a–nitroso–b–naphthol, cupron, etc.
-- NCERT Class 12 Chemistry, Ch. 5, p. 136Hardness of water is estimated by simple titration with Na2EDTA. The Ca 2+ and Mg 2+ ions form stable complexes with EDTA. The selective estimation of these ions can be done due to difference in the stability constants of calcium and magnesium complexes.
-- NCERT Class 12 Chemistry, Ch. 5, p. 136Some important extraction processes of metals, like those of silver and gold, make use of complex formation. Gold, for example, combines with cyanide in the presence of oxygen and water to form the coordination entity [Au(CN)2] – in aqueous solution. Gold can be separated in metallic form from this solution by the addition of zinc.
-- NCERT Class 12 Chemistry, Ch. 5, p. 136Similarly, purification of metals can be achieved through formation and subsequent decomposition of their coordination compounds.
-- NCERT Class 12 Chemistry, Ch. 5, p. 136For example, impure nickel is converted to [Ni(CO)4], which is decomposed to yield pure nickel.
-- NCERT Class 12 Chemistry, Ch. 5, p. 137Coordination compounds are of great importance in biological systems. The pigment responsible for photosynthesis, chlorophyll, is a coordination compound of magnesium. Haemoglobin, the red pigment of blood which acts as oxygen carrier is a coordination compound of iron. Vitamin B12, cyanocobalamine, the anti– pernicious anaemia factor, is a coordination compound of cobalt. Among the other compounds of biological importance with coordinated metal ions are the enzymes like, carboxypeptidase A and carbonic anhydrase (catalysts of biological systems).
-- NCERT Class 12 Chemistry, Ch. 5, p. 137Coordination compounds are used as catalysts for many industrial processes. Examples include rhodium complex, [(Ph3P)3RhCl], a Wilkinson catalyst, is used for the hydrogenation of alkenes.
-- NCERT Class 12 Chemistry, Ch. 5, p. 137Articles can be electroplated with silver and gold much more smoothly and evenly from solutions of the complexes, [Ag(CN)2] – and [Au(CN)2] – than from a solution of simple metal ions.
-- NCERT Class 12 Chemistry, Ch. 5, p. 137In black and white photography, the developed film is fixed by washing with hypo solution which dissolves the undecomposed AgBr to form a complex ion, [Ag(S2O3)2] 3–.
-- NCERT Class 12 Chemistry, Ch. 5, p. 137Key Fact
Medicine: chelate therapy (D-penicillamine, desferrioxime B), EDTA for lead poisoning, cis-platin
There is growing interest in the use of chelate therapy in medicinal chemistry. An example is the treatment of problems caused by the presence of metals in toxic proportions in plant/animal systems. Thus, excess of copper and iron are removed by the chelating ligands D–penicillamine and desferrioxime B via the formation of coordination compounds. EDTA is used in the treatment of lead poisoning. Some coordination compounds of platinum effectively inhibit the growth of tumours. Examples are: cis–platin and related compounds.
-- NCERT Class 12 Chemistry, Ch. 5, p. 137Applications Coordination: NEET previous year questions (PYQs) with answers
19 questions in Coordination Compounds
No question in our NEET 2020–2025 set targets this topic directly.
More in Coordination Compounds: 2 exam traps and mistakes · 2 formulas · 3 question patterns from its other lessons.
Sources
NCERT refs: Class 12 Chemistry Chapter 5, p.136 | Class 12 Chemistry Chapter 5, p.137
Page numbers are the ones printed in the current NCERT textbook (2023 rationalised edition), unless marked pre-2023. The books are free at ncert.nic.in.
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