Preparing for Class 12 Chemistry exams involves understanding key concepts and practicing with previous years' questions (PYQs). These questions are valuable because they give insights into the exam pattern, important topics, and types of questions frequently asked.
COHSEM QUESTION PAPER
(2010 – 2023)
UNIT – I
(SOLID STATE)
COHSEM 2010
Q1. Thallium chloride, TlCl crystallises in a simple cubic lattice or face centred cubic lattice of Cl − ions
Tl + ions in the holes. If the density of the solid is 9.00gcm−3 and edge of the unit cell is 3.85 × 10 −8 cm ,
what is the unit cell geometry? ( At. mass of Tl = 2.4.5µ ) . 2
Q2. Explain the structural differences between three types of cubic crystals. 3
COHSEM 2011
Q1. Name a compound having the body – centred cubic unit cell crystal lattice. 1
Q2. A uni – univalent ionic crystal AX is composed of the following radii (arbitrary units):
A+ X−
.
1.0 2.0
Assuming that ions are hard spheres, predict giving reasons, whether the crystal will have the sodium
chloride or caesium chloride structure. Calculate the volume of the unit cell. 2
COHSEM 2012
Q1. All the ions of NaF and MgO have the same number of electrons and intermolecular distances are
about the same (234 pm and 215 pm). Then, why are the melting point of NaF 9920 C and ( )
MgO ( 26420 C ) so different. 1
2
Q2. In a compound, atoms of element Y form ccp lattice those of element X occupy rd of tetrahedral void.
3
The formula of the compound will be
A. X 2Y B. X 3Y4 C. X 4Y3 D. X 3Y2 . 1
Q3. How can you determine the atomic mass of an unknown metal if you know its density and the
dimensions of its unit cell? Explain your answer.
COHSEM 2013
Q1. What type of imperfection occurs when a crystal misses equal number of captions and anions from
their normal lattice site. 1
0
Q2. Iron (II) oxide, FeO , crystallizes in the cubic from with the edge length of 5 A of each unit cell. If the
density of the crystal is 4gcm−3 , calculate the number of Fe 2+ and O −2 ions present in each unit cell.
(Given N A = 6.022 ×1023 ). 3
COHSEM 2014
Q1. Some polar crystals when heated produce small electrical current. The phenomenon is called
A. Ferroelectricity B. Anti – ferroelectricity
C. Pyroelectricity D. Piezoelectricity. 1
Q2. Name the types of semiconductors produced when Germanium ( Ge ) is doped separately with Boron
( B ) and aluminium ( Al ) . Which one will be a better semi conductor and why? 1+1+1=3
COHSEM 2015
Q1. Define (i) unit cell (ii) coordination number, in relation to crystalline solids. 2
Q2. Explain the Schottky defect in Stoichiometric crystals. What are the consequences of Schottky and
Frenkel defects in crystals? 3
COHSEM 2016
Q1. What is a semiconductor? Mention the type of semiconductor obtained when silicon is doped with
arsenic. 1+1=2
Q2. The density of a face centred cubic element (atomic mass = 60.2 amu) is 6.25 g cm−3 . Calculate the
edge of unit cell. 2
1|ctzx
, COHSEM 2017
Q1. What is Schottky defect? 1
Q2. A ferromagnetic substance becomes a permanent magnet when it is placed in a magnetic field
because ______. 1
A. all the domains get oriented in the direction of magnetic field.
B. all the domains get oriented in the direction opposite to the direction of magnetic field.
C. domains get oriented randomly.
D. domains are not affected by magnetic field.
Q3. Write two differences between crystalline solids and amorphous solids. 2
COHSEM 2018
Q1. What are rectifiers and transistors? How are they made? 2
Q2. Aluminium metal crystallises in a cubic structure in which the edge of the unit cell is 405 pm.
Determine the type of unit cell if the density of Al is 2.7 g cm−3 . 2
COHSEM 2019
Q1. What is meant by imperfections in solids? What is the effect of interstitial defect on the density of a
solid? 2
Q2. Elements A and B form a crystalline compound. In the crystal atoms of element A form cubic closed
packed structure and atoms of element B occupy 1/ 3 rd of tetrahedral voids. Calculate the formula of
the compound. 2
COHSEM 2020
Q1. KBr crystal does not show Frenkel defect. Give reason. 1
Q2. Atoms of element B (as anions) make CCP and those of element A (as cations) occupy all the octahedral
voids. Predict the formula of the compound. 1
+ −
Q3. What type of detect can arise when ( as SrCl2 ) is added as impurity in ionic solid N a C l . Justify your
answer. 2
COHSEM 2022
Q1. A compound formed by element A and B has the A atoms forming a ccp lattice and the B atoms occupy
half of the tetrahedral voids. What is the co-ordination number of B atoms in the crystal? 1
Q2. Classify solids into different types on the basis of the arrangement of constituent particles. 2
Q3. Platinum crystallizes in a face-centered cubic lattice with all atoms at the lattice points. Calculate the
mass of a unit-cell of the metallic crystal (Atomic mass of platinum = 195 amu). 2
COHSEM 2023
Q1. a) An element having atomic mass 80 g mol −1 adopts face centre cubic structure. Calculate the
number of unit cells present in 8 g of the element.
b) NaCl crystal becomes yellowish when heated with sodium vapour. Explain what effect would be
observed on the density of the crystal? 3+2=5
OR
Q1. a) An alloy of three metals A, B and C has the A atoms forming ccp lattice while B atoms occupy the
edge centres and C atoms occupy the body centres. Establish the formula of the alloy.
b) Germanium is an intrinsic semiconductor. How can you increase its conductivity? 3+2=5
*****
UNIT – II
SOLUTION
COHSEM 2010
Q1. i) A solution contains 75% water and 25% ethanol by mass. Calculate the mole fractions of ethanol and
molality of the solution.
ii) Calculate the osmotic pressure at 273K of a 5% solution of urea. (molecule mass = 60)
Given: R =0.0821 lit atm/deg/mol.
iii) Will equimolar solutions of sodium chloride and glucose be isotonic or not? Give reason in support
of your anser. 2+2+1=5
COHSEM 2011
Q1. Explain why equimolar aqueous solution of sodium chloride and sodium sulphate are not isotonic. 2
2|ctzx
, Q2. An aqueous solution freezes at 272.07K while pure water freezes at 273K. Determine the molality and
boiling point of the solution. Given K f = 1.86 K / m; Kb = 0.512 K / m . 3
COHSEM 2012
Q1. A solution containing 5 g of solute dissolved in 250 cm3 of water gave an osmotic pressures of
4.5 atm at 27 0 C . Calculate the boiling point of the solution. The molal elevation constant for water is
0.520 C per 1000 g . 5
COHSEM 2013
Q1. Draw the vapours pressure temperature curves showing the depression of freezing point of the solution
of a non – volatile solute from that of the solvent. In the graph mark the points which represent the
freezing point of pure solvent and solution. 2
Q2. Acetic acid dimerises in benzene. When 1.65g of acetic acid is dissolved in 100g of benzene the boiling
point of raised by 0.360 C . Calculate the Van’t Hoff factor and degree of association of the acid in
benzene (Molal elevation constant of benzene is (Molal elevation constant of benzene is
2.57 K Kg mol −1 ). 3
COHSEM 2014
Q1. Which one of the following concentrations is not affected by temperature? 1
A. Normality B. Molality C. Molarity D. Formality.
Q2. Establish a relationship between mole fraction of solute and molality of a binary dilute solution. 2
Q3. Osmotic pressure of a solution containing 3.5 gm of the protein per 50 c.c of a solution is 25 mm Hg at
37 0 C . Calculate the molar mass of the protein. 2
COHSEM 2015
Q1. A solution of glucose in water is labelled as 10% w/w. What would be the molality and mole fraction of
each components in the solution? 2
Q2. Explain, with the help of vapour pressure curves, why a solution containing non – volatile solute has
higher boiling point than pure solvent. 3
COHSEM 2016
Q1. Which of the following solutions will have the highest boiling points? 1
A. 1% NaCl B. 1% BaCl2 C. 1% glucose D. 1% surcrose .
Q2. State: A) Raoult’s law for Volatile solid. B) Van’t Hoff factor. 2
0
Q3. Calculate the Osmotic pressure of 5% solution of cane sugar (sucrose) at 1.5 C
( R = 0.082 litre atm K −1
mol −1
). 2
COHSEM 2017
Q1. What are the characteristic of ideal solution? 1
Q2. Why does vapour pressure of a liquid decrease when a non – volatile solute is added into it? 1
Q3. What is Raoult’s law? The vapour pressure of a 5% aqueous solution of non – volatile organic
substance at 375K is 745 mm . Calculate the molar mass of the solute. 3
COHSEM 2018
Q1. Calculate the molality and mole fraction of a solution containing 2.5 g ethanoic acid ( CH 3COOH ) in
75 g of benzene. 2
Q2. What is an ideal solution? What are the necessary conditions for a solution to be ideal? 3
COHSEM 2019
Q1. Why is Henry’s law not applicable to dissolution of HCl gas in water? 1
Q2. Define Van’t Holf factor. What would be the value of Van’t Hoff factor for a diluted aqueous solution of
Na2 SO4 ? 2
Q3. Calculate elevation in boiling point when 300 g of urea is dissolved in 2500 g of water
( K for water = 0.52Kg mol ) .
b
−1
2
3|ctzx
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