UPSC Previous Year Question Paper 2026 – Combined Geo-Scientist (Main) Examination | SNCS-A-CMS Chemistry Paper-I | Free PDF Download
UPSC Previous Year Question Paper 2026 – Combined Geo-Scientist (Main) Examination | SNCS-A-CMS Chemistry Paper-I | Free PDF Download
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Combined Geo- Scientist (Main) Examination, 2026 SNCS- A- CMS
CHEMISTRY
PAPER- I
Time Allowed : Three Hours
Maximum Marks : 200
QUESTION PAPER SPECIFIC INSTRUCTIONS
Please read each of the following instructions carefully before attempting questions
There are ELEVEN questions divided under SIX Sections.
Candidate has to attempt SIX questions in all.
The ONLY question in Section- A is compulsory.
Out of the remaining TEN questions, the candidate has to attempt FIVE, choosing ONE from each of the other Sections B, C, D, E and F.
The number of marks carried by a question/part is indicated against it.
Neat sketches are to be drawn to illustrate answers, wherever required. These shall be drawn in the space provided for answering the question itself.
Unless otherwise mentioned, symbols and notations have their usual standard meanings.
Assume suitable data, if necessary, and indicate the same clearly.
Attempts of questions shall be counted in sequential order. Unless struck off, attempt of a question shall be counted even if attempted partly.
Any page or portion of the page left blank in the Question- cum- Answer (QCA) Booklet must be clearly struck off.
Answers must be written in ENGLISH only.
UPSC Previous Year Question Paper 2026, UPSC Combined Geo-Scientist Main Examination 2026, Combined Geo-Scientist Main Examination Chemistry Paper 1, UPSC Geo Scientist Chemistry Paper 1 2026, UPSC Combined Geo Scientist Chemistry Question Paper, UPSC Geo
[Combined Geo-Scientist (Main) Examination – Previous Year Question (Chemistry Paper-I):
Q. Explain the crystal field splitting in octahedral and tetrahedral complexes. How does it account for the colours of transition metal complexes?]
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Answer all of the following questions : 5×10=50
(a) Zinc blende and wurtzite are two different naturally occurring minerals, having same cation/anion in unary composition, but crystallizes in two different forms. Rationalize the observation with radius of cation (74 pm or 88 pm) and anion (184 pm).
(b) Intensities of spin-forbidden transitions are greater for 4d4d - or 5d5d -series metal complexes than for 3d3d -series metal complexes. Explain it.
(c) Phosphate ions are abundant in cells. The typical total phosphate concentration in a cell [HPO42−]+[H2PO4−][HPO42−]+[H2PO4−] is 2.0×10−2M2.0×10−2M . What are the concentrations of HPO42−HPO42− and H2PO4−H2PO4− at pH7⋅40?pH7⋅40? (Given : pKapKa for H2PO4−H2PO4− ion is 7-20, which is very close to the normal pH in the body)
H2PO4−(aq)+H2O(l)⇌H3O+(aq)+HPO42−(aq)H2PO4−(aq)+H2O(l)⇌H3O+(aq)+HPO42−(aq)
(d) Find the concentration of added Ag+Ag+ to just start precipitation of AgClAgCl in a 1.0×10−3M1.0×10−3M solution of NaCl, if KspKsp of AgClAgCl at 25∘C25∘C is 1.0×10−101.0×10−10
[Combined Geo-Scientist (Main) Examination – Previous Year Question (Chemistry Paper-I):
Q. What are masking agents? Explain their role in complexometric titrations with suitable examples.]
(e) Distinguish between iodometry and iodimetry with examples.
(f) Give at least two examples of masking agents. How are they used in the estimation of Ca2+Ca2+ ion, if the solution contains ions like Ni2+Ni2+ or Ag+Ag+ ? Explain with suitable equations.
(g) Identify the first row transition metal in the following organometallic compounds obeying EAN rule :
[M(CO)3(PPh3)]−[M(CO)3(PPh3)]− [M(m1−C5H5)(m5−C5H5)(CO)2][M(m1−C5H5)(m5−C5H5)(CO)2] [M(m5−C5H5)(CO)3]2[M(m5−C5H5)(CO)3]2
(h) Find the values of aa and bb in the following reactions of radioactivity :
(i)21082Pb−β2X(i)82210Pb−2βX(ii)22688Ra−α2Y(ii)88226Ra−2αY
Predict the position of the daughter element in the periodic table.
[Combined Geo-Scientist (Main) Examination – Previous Year Question (Chemistry Paper-I):
Q. State and explain the Fajans rules for covalent character in ionic compounds.]
(i) Account for the similar electronic spectra of Eu3+Eu3+ complexes with various ligands and the variation of electronic spectra of Am3+Am3+ complexes as the ligand is varied.
Calculate the minimum moles of urea required for complete precipitation of 0⋅02mol0⋅02mol of Fe3+Fe3+ as Fe(OH)3Fe(OH)3
SECTION-B
Attempt any one question.
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(a) Which one, SrCl2SrCl2 or CdCl2CdCl2 , follows fluorite structure? Explain with suitable pictorial diagram. [Ionic radii (in pm) : Sr2+−113Sr2+−113 ; Cd2+−97Cd2+−97 ; Cl−−181Cl−−181 ] 10
(b) The spectrum of [Cr(NCS)6]3−[Cr(NCS)6]3− has a very weak band near 16000cm−116000cm−1 , a band at 17700cm−117700cm−1 with ϵmax=160dm3mol−1cm−1ϵmax=160dm3mol−1cm−1 , a band at 23800cm−123800cm−1 with ϵmax=130dm3mol−1cm−1ϵmax=130dm3mol−1cm−1 , a very strong band at 32400cm−132400cm−1 . Assign these transitions, using selection rule considerations. 10
(c) Nickel and palladium both form complexes of the general formula M(PR3)2Cl2M(PR3)2Cl2 . The nickel(II) compound is paramagnetic, whereas the palladium(II) compound is diamagnetic.
(i) Explain the magnetic properties of these compounds.
(ii) How many isomers of each compound are expected? 10
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(a) Among the mixed metal oxides NiCr2O4NiCr2O4 and NiCo2O4NiCo2O4 , one of the oxides is structurally dissimilar to MgAl2O4MgAl2O4 . Explain. 10
(b) cis- PtCl2(PET3)2PtCl2(PET3)2 is stable in benzene solution. However, small amount of free triethylphosphine catalyzes establishment of an equilibrium with the transisomer :
cis−PtCl2(PET3)2⇌trans−PtCl2(PET3)2cis−PtCl2(PET3)2⇌trans−PtCl2(PET3)2
For the conversion of cis to trans in benzene at 25∘C25∘C , ΔH∘=10.3kJmol−1ΔH∘=10.3kJmol−1 and ΔS∘=55.6Jmol−1K−1ΔS∘=55.6Jmol−1K−1 .
(i) Calculate the equilibrium constant for this isomerization.
(ii) Which isomer has higher bond energy? Explain briefly.
(iii) Why is free triethylphosphine necessary to catalyze the isomerization? 10
[Combined Geo-Scientist (Main) Examination – Previous Year Question (Chemistry Paper-I):
Q. Discuss the lanthanide contraction. How does it influence the chemistry of lanthanides?]
(c) (i) Give a balanced equation for the reaction of lanthanoids with aqueous acid. Justify your answer.
(ii) Name the two lanthanoids that have greatest tendency to deviate from the usual positive oxidation state and correlate this deviation with electronic structure. 10
SECTION-C
Attempt any one question.
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(a) Suggest indicators in the following titration :
0- 1 M ethylenediamine and HCl
Given :
(Assume the quantitative formation of mono- and di- protonated species) 10
(b) What is homogeneous precipitation? Discuss its advantages over direct precipitation. 10
(c) Explain the working principle of redox indicators with example. 10
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(a) Suggest why the pertechnetate ion is red, permanganate ion is purple and manganate ion is green. Describe the most likely absorption that gives rise to colours in these ions. Also include energy level sketch showing how d-orbitals on metals interact with oxide orbitals to form molecular orbitals. 10
(b) Describe the experimental procedure for gravimetric estimation of nickel in grams per litre with necessary calculation steps. 10
(c) How is standard electrode potential (E∘)(E∘) of Mn7+/Mn2+Mn7+/Mn2+ determined experimentally through potentiometric titration of standard KMnO4KMnO4 versus Mohr's salt solution? 10
SECTION-D
Attempt any one question.
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(a) Consider the following equilibrium :
KCN(excess)+AgNO3⇌I+JKCN(excess)+AgNO3⇌I+J
Find the compounds II and JJ , and draw the crystal field splitting diagram (with labels and filled electrons of orbitals) of silver complex (I)(I) . 10
[Combined Geo-Scientist (Main) Examination – Previous Year Question (Chemistry Paper-I):
Q. Derive the Nernst equation for a galvanic cell and explain its significance.]
(c) Derive Nernst equation for the cell potential of silver-copper cell. 10
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(a) An aqueous solution of CuSO4CuSO4 and CdSO4CdSO4 was combined with excess KCN solution and then H2SH2S (gas) was passed through the solution mixture to obtain an insoluble yellow-coloured precipitate. Find the products and give reasons, along with equations, as to why only one of the metal compounds is formed as precipitate. 10
(b) Phenol (C6H5OH)(C6H5OH) is a weak organic acid. Suppose 0.515g0.515g of the said compound is dissolved in enough water to make 125 mL125 mL of the solution. The resulting solution is titrated with 0.123M0.123M NaOH. [Given : KaKa (Phenol) =1.3×10−10=1.3×10−10 at 25∘C25∘C ]
(i) What is the pH of the original solution of phenol?
(ii) What are the concentrations of all of the following ions at the equivalence point?
Na+,H3O+,OH−andC6H5O−Na+,H3O+,OH−andC6H5O−
(iii) What is the pH of the solution at the equivalence point? 20
SECTION-E
Attempt any one question.
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(a) In the following equilibrium
find M assuming that the magnetic moments of [MCl4]2−[MCl4]2− and [M(H2O)6]2+[M(H2O)6]2+ are 3⋅873⋅87 BM and ∼4⋅1∼4⋅1 BM respectively. What will be the colour of these complexes, and why is the equilibrium shifted upon addition of Zn2+Zn2+ or Ca2+Ca2+ ions to the reaction medium? 10
(b) Among the following organometallic compounds, which one will have the maximum number of bonds between the two metal centres, and a least number of dd -electrons at the metal centre?
(n5−C5H5)2Ni2(CO)2(n5−C5H5)2Ni2(CO)2VI2(CO)12VI2(CO)12
\mathrm{(n}^{5}\mathrm{-C_{5}H_{5})_{2}Re_{2}(\mu_{2}\mathrm{-CO})_{3}}}
(n5−C5H5)2Re2(μ2−O)2(O)2(n5−C5H5)2Re2(μ2−O)2(O)2
[Combined Geo-Scientist (Main) Examination – Previous Year Question (Chemistry Paper-I):
Q. What is isotope dilution analysis? Explain its application in the determination of trace elements.]
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(a) How is isotope dilution method used to determine the amount of an element present in a sample? 10
(b) Find the products (K to N) in the following reaction and show the metal to ligand bond connectivities, only for organometallic species :
[Mn(CO)5]−+C3H5Cl⟶K+L[Mn(CO)5]−+C3H5Cl⟶K+L ↓Δorhv↓Δorhv M+NM+N
(c) For each of the compounds [Co(bipy)3]2+[Co(bipy)3]2+ and [Co(NH3)6]2+[Co(NH3)6]2+ , answer the following :
(i) Find the ground state term symbol.
(ii) Use the Tanabe-Sugano diagram to identify the predicted spectral bands.
(iii) Do you expect broad or narrow absorption bands in the visible and UV regions?
(iv) Sketch an MO energy level diagram for each. 10
SECTION-F
Attempt any one question.
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(a) The amount of C-14 isotope in a piece of wood is found to be one-sixth of its amount present in a fresh piece of wood. Calculate the age of the wood. Given that half-life of C-14 = 5577 years. 10
(b) The stretching frequency values of both starting material and the products are given below in the reversible reaction. Find M which belongs to the principal quantum number of n=5n=5 and draw both the structures :
[m5−C5H5]M(CO)3]2⇌[m5−C5H5]M(CO)2]2+2CO[m5−C5H5]M(CO)3]2⇌[m5−C5H5]M(CO)2]2+2CO vˉ=1960,1915cm−1vˉ=1889,1859cm−1vˉ=1960,1915cm−1vˉ=1889,1859cm−1
(c) Barium ion estimation can be done using EDTA directly, whereas aluminium ion estimation cannot be performed directly with EDTA. Why? 10
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(a) Explain the construction and working principle of Geiger-Müller counter with diagram. 10
[Combined Geo-Scientist (Main) Examination – Previous Year Question (Chemistry Paper-I):
Q. Discuss the stability of metal carbonyls on the basis of synergic bonding.]
(b) Find the correct pair(s) of stable organometallic compounds and rationalize the answer :
(i) Ti(CO)4Ti(CO)4 , Ni(CO)4Ni(CO)4
(ii) [Fe(CO)6]2+[Fe(CO)6]2+ , [Mn(CO)6]+[Mn(CO)6]+
(iii) Co(CO)5Co(CO)5 , Fe(CO)5Fe(CO)5
(iv) [V(CO)6]−[V(CO)6]− , [Co(CO)4]−[Co(CO)4]−
(c) One of the rarest minerals cristobalite of silica forms basic core unit similar to that of one of the allotropes of carbon. Draw this basic core unit of silica and provide a rationale as to why carbon analogue does not form such network type structure. 10
[Combined Geo-Scientist (Main) Examination – Previous Year Question (Chemistry Paper-I):
Q. Explain the structure and bonding of ferrocene. Why is it aromatic?]
Combined Geo Scientist Main Examination Chemistry 1 Free PDF download link
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