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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.
SNCS-A-CMS/72 1 [ P.T.O.
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OOS gesN ANS
( Compulsory Section )
1. Answer all of the following questions : 5x10=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 4d- or 5d-series metal
complexes than for 3d-series metal complexes. Explain it.
(c) Phosphate ions are abundant in cells. The typical total phosphate
concentration in a cell [HPO?~]+[HjPO4] is 2:0x10"? M. What are the
concentrations of HPO?” and HjPOj at pH 7-40? (Given : pK, for H2POj ion is
7:20, which is very close to the normal pH in the body)
HPO; (aq) +H20 (0) = H30* (aq) + HPO2” (aq)
(a) Find the concentration of added Ag* to just start precipitation of AgCl in a
1-0x107* M solution of NaCl, if Ky) of AgCl at 25 °C is 1-.0x107!°.
(e) Distinguish between iodometry and iodimetry with examples.
() Give at least two examples of masking agents. How are they used in the
estimation of Ca?* ion, if the solution contains ions like Ni2* or Ag* ? Explain
with suitable equations.
(g) Identify the first row transition metal in the following organometallic
compounds obeying EAN rule :
() [M(CO)3(PPh3)]"
(i) [M(n'-C5Hs)(n°-CsH5)(CO}9]
(it) [M(n°-CsHs)(CO)g]2
(h) Find the values of a and b in the following reactions of radioactivity :
() 79pp—-B_, ex
(i) 72Spqg—=* , ay
Predict the position of the daughter element in the periodic table.
SNCS-A-CMS/72 2
()
0)
(a)
(b)
(c)
(a)
(b)
Account for the similar electronic spectra of Eu°* complexes with various
ligands and the variation of electronic spectra of Am** complexes as the ligand
is varied.
Calculate the minimum moles of urea required for complete precipitation of
0-02 mol of Fe°* as Fe(OH)3.
SECTION—B
Attempt any one question.
Which one, SrClz or CdClg, follows fluorite structure? Explain with suitable
pictorial diagram. [Ionic radii (in pm) : Sr?*—113; Cd?*—97; Cl"-—181]
The spectrum of [Cr(NCS),6]>- has a very weak band near 16000 cm™!, a band
at 17700 cm~! with €max =160 dm? mol! cm™!, a band at 23800 cm™ with
Emax =130 dm? mol! cm7}, a very strong band at 32400 cm7!. Assign these
transitions, using selection rule considerations.
Nickel and palladium both form complexes of the general formula M(PR3)2Cl4.
The nickel(II) compound is paramagnetic, whereas the palladium(II) compound
is diamagnetic.
(i) Explain the magnetic properties of these compounds.
(i) How many isomers of each compound are expected?
Among the mixed metal oxides NiCr20, and NiCo20,4, one of the oxides is
structurally dissimilar to MgAl,0,. Explain.
cis-PtCl,(PEt3)2 is stable in benzene solution. However, small amount of free
triethylphosphine catalyzes establishment of an equilibrium with the trans-
isomer :
cis-PtCl,(PEt3)2 = trans-PtCl,(PEt3)2
For the conversion of cis to trans in benzene at 25 °C, AH°=10-3 kJ mol”! and
AS°=55-6 J mol! K7}.
(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?
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SNCS-A-CMS/72 3 [P:T:0:
(c) (i) Give a balanced equation for the reaction of lanthanoids with aqueous
acid. Justify your answer.
(i) 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.
4. (a) Suggest indicators in the following titration :
0:1 M ethylenediamine and HCl
Given :
K,, =8-5x10°
m1 _g } at 25 °C
Kp, =2-7x10
(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
5. (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
(Cc) How is standard electrode potential (E°) of Mn’*/Mn?* determined
experimentally through potentiometric titration of standard KMnO, versus
Mohr’s salt solution? 10
SECTION—D
Attempt any one question.
6. (a) Consider the following equilibrium :
KCN (excess) + AgNO, = I+J
Find the compounds J and J, and draw the crystal field splitting diagram (with
labels and filled electrons of orbitals) of silver complex (J). 10
SNCS-A-CMS/72 4
(c)
(b)
8. (a)
(b)
Derive an equation having relation between hydrolysis constant and degree of
hydrolysis of a salt of weak base and strong acid.
Derive Nernst equation for the cell potential of silver-copper cell.
An aqueous solution of CuSO, and CdSO,4 was combined with excess KCN
solution and then H2S (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.
Phenol (CgH;OH) is a weak organic acid. Suppose 0-515g of the said
compound is dissolved in enough water to make 125 mL of the solution.
The resulting solution is titrated with 0-123 M NaOH. [Given : K, (Phenol)
=1-3x107!° at 25 °C]
(i) What is the pH of the original solution of phenol?
(i) What are the concentrations of all of the following ions at the equivalence
point?
Na*, H3;0*, OH” and C,H507
(iii) What is the pH of the solution at the equivalence point?
SECTION—E
Attempt any one question.
In the following equilibrium
Zn2*
[MCl4]?- + 6H20 — [M(H20)¢]2* + 4C17
a
find M assuming that the magnetic moments of [MC1,]2~ and [MH20)_]?* are
3-87 BM and ~ 4-1 BM respectively. What will be the colour of these complexes,
and why is the equilibrium shifted upon addition of Zn2* or Ca?* ions to the
reaction medium?
Among the following organometallic compounds, which one will have the
maximum number of bonds between the two metal centres, and a least
number of d-electrons at the metal centre?
() (n°-CsHs)2 Nig (CO)
(i) V2(CO)2
(iii) (n°-C5Hs)o Reo (42-CO)3
(iv) (n°-CsHs)o Reg (2-O)2 On
SNCS-A-CMS/72
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20
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(c)
9. (a)
()
(c)
10. (a)
()
(c)
11. (a)
Calculate the half-life period of radium-226, if 0-001 kg of it emits 3-7x10!°
alpha particles per second.
How is isotope dilution method used to determine the amount of an element
present in a sample?
Find the products (K to N) in the following reaction and show the metal to
ligand bond connectivities, only for organometallic species :
[Mn(CO)s5]" + C3H5Cl1 —> K+ L
| A or hv
M+ WN
For each of the compounds [Co(bipy)3]?* and [Cof(NH3)6]?*, 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.
SECTION—F
Attempt any one question.
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.
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=5 and draw both the structures :
[m>-CsHs)M(CO)3]2_ = [(n°-CsHs)M(CO)g]2 + 2CO
¥ =1960, 1915cm™! ¥ =1889, 1859 cm7!
Barium ion estimation can be done using EDTA directly, whereas aluminium
ion estimation cannot be performed directly with EDTA. Why?
Explain the construction and working principle of Geiger-Miuller counter with
diagram.
SNCS-A-CMS/72 6
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(b) Find the correct pair(s) of stable organometallic compounds and rationalize the
answer :
8 Ti(CO)4, 100),
(ii) ` [୧୧(୦୦)6]2* , [01000)61
| (1) (00. ₹૦(૦૦ડ
| fix) [906], 96941 10
(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
kkk
SNCS-A-CMS/72 7 26BS—240
