1st PUC Chemistry Model Question Paper 1 with Answers

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Karnataka 1st PUC Chemistry Model Question Paper 1 with Answers

Time: 3.15 Hours
Max Marks: 100

Instructions:

1. The question paper has four parts A, B, C and D, All parts are compulsory.
2. Write balanced chemical equations and draw labelled diagram wherever required.
3. Use log tables and simple calculators if necessary (Use of scientific calculators is not allowed].

Part -A 

I. Answer ALL of the following (each question carries one mark: ( 10 × 1 = 10 )

Question 1.
What is the SI unit of density?
Answer:
kg m^-3.

Question 2.
What are real gases?
Answer:
Real gases are those which do not follow all the gas laws at all temperature and pressure.

Question 3.
At what condition K_p = K_c in a equilibrium?
Answer:
K_p = (RT)^∆n When ∆n = 0 i.e., number of moles of the gaseous products = number of moles of the gaseous reactants, e.g. H₂(g) + I₂(g) 2HI_g – K_p = K_c

Question 4.
What are chalcogens?
Answer:
Oxygen family or 16^th group elements.

Question 5.
What is the oxidation state of ‘N’ in HNO₃?
Answer:
N = +5

Question 6.
Alkali metals are soft metals. Why?
Answer:
Due to low ionisation enthalpy.

Question 7.
What is dry ice?
Answer:
Solid CO₂ is called dry’ ice. It is called dry ice because it directly changes into gaseous state without becoming liquid.

Question 8.
What is the chemical formula of borax?
Answer:
Na₂B₄O₇.10H₂O or boric acid / diborane.

Question 9.
Write the TUPAC name of (CH₃)₂CHCH₂OH.
Answer:
2-Methy Propanol.

Question 10.
Complete the following reaction.

Answer:

Part – B

II. Answer any five of the following questions carrying two marks: ( 5 × 2 = 10 )

Questioin 11.
Calculate the amount of water (g) produced by the combution of 16g of methane.
Answer:
CH(_4g) + 2O_2(g) → CO_2(g) + 2H₂O₍₁₎
1 mole = 16 g of CH₄ gives 2 moles of H₂O i.e., 2 × 18 = 36 g of H₂O.

Question 12.
Give the expressions for,
(a) van der Waals equation for ‘n’ moles of a gas. (b) Compressibility factor (Z).
Answer:
(a) (P + \(\frac{\mathrm{an}^{2}}{\mathrm{V}^{2}}\) )(V – nb )= nRT
(b) In case of real gases, the deviation from ideal behaviour can be measured in terms of compressibility factor (Z).
Z is mathematically defined as Z = \(\frac{\mathrm{PV}}{\mathrm{nRT}}\)

Question 13.
What is an ionic bond? Explain the formation of ionic bond with an example.
Answer:
An ionic bond is the electrostatic force of attraction between oppositely charged ions. OR A chemical bond formed by complete transfer of one or more electrons from outermost orbit of electropositive atom to outer most orbit of electronegative atom.
NaCl is formed by the combination of sodium and chlorine. An atom of sodium transfer an electron to chlorine and form sodium ion (positive ion).

The chlorine atom gain, one electron from sodium and form chloride ion (negative ion).

Thus both Na⁺ and Cl^– ions are held by electrostatic force of attraction to form ionic molecule NaCl.

Question 14.
Give any two similarities between lithium and magnesium due to diagonal relationship.
Answer:
Both lithium and magnesium have small size and high charge density. The electronegativities of Li is 1.0 and Mg is 1.2. They are low and almost same. Their ionic radii are similar. Hence they show similarities which is known as diagonal relationship between first element of a group with the second element in the next higher group.

Question 15.
What happens when formic acid is heated with cone. H₂SO₄? Give equation.
Answer:
Pure carbon monoxide is formed by the dehydration of formic acid with cone. H₂SO₄

Question 16.
Explain Wurtz reaction with an example.
Answer:
When alkyl halides are heated with sodium metal in ether medium higher alkanes are formed. This reaction is known as Wurtz reaction and employed for the synthesis of higher alkanes containing even number of carbon atoms.

Question 17.
Describe the preparation of alkenes by dehydration of alcohols.
Answer:
When ethanol is heated with concentrated sulphuric acid, they undergo dehydration to give alkenes.

Question 18.
How is ‘Ozone layer’ formed in the stratosphere? Name a chief chemical that causes its depletion.
Answer:
(a) Formation of ozone in upper atmosphere: The UV radiations of very’ short wavelength • (< 250 nm) have sufficient energy to cause photo-dissociation of oxygen molecules into
oxygen atoms. Highly reactive oxygen atoms combine with oxygen molecules to form ozone.

(b) Breakdown of ozone in lower atmosphere: The ozone thus formed is an excellent absorber of ultraviolet radiations and it breakdown into dioxygen and an oxygen atom.

(c) (i) Hydroxyl free radical (ii) NO, NO2₂ etc
(iii) Chloro fluoro carbons (CFC’s).

Part – C

III. Answer any FIVE of the following questions carrying THREE marks: ( 5 × 3 = 15 )

Question 19.
(a) Define electron gain enthalpy. How does it vary along a period?
(b) Name the most electronegative element in the periodic table.
Answer:
(a) It is the amount of heat energy liberated during the addition of e to neutral gaseous isolated atom. .
Electron gain enthalpy increases along the periods from left to right in each period.
(b) Fluorine is the most electronegative (EM – 4.0) element.
Ceasium is the least electronegative (EN = 0.7) element.

Question 20.
Explain the structure of ethane (C₂H₆) molecule on the basis of hybridization.
Answer:
The molecular formula of ethane is C₂H₆
Electronic configuration of C is ground state – 1s²2s²2p²
Electronic configuration of C is excited state – 1s²2s¹2p³
Valence orbital representation

Question 21.
Give the electronic configuration, bond order and magnetic property of carbon molecule based on molecular orbital theory.
Answer:
Atomic number of carbon is 6 : 1s², 2s², 2P²

(a) Electronic configuration of carbon molecule is KK σ 2S²σ * 2S² π2p²_x π2p²_y
(b) Bond order = \(\frac{6-2}{2}\) = 2
(c) Magnetic property = diamagnetic.

Question 22.
Define “Hydrogen bond”. Give an example for the molecule having
(a) Intermolecular hydrogen bond (b) Intramolecular hydrogen bond.
Answer:
Electrostatic force of attraction that exists between hydrogen atom of one molecule and electronegative atom of same or other molecule is called hydrogen bond.

Type of hydrogen bond:

1. Intermolelcular hydrogen bond.
2. Intramolecular hydrogen bond.

1. Intermolecular hydrogen bond: Electrostatic force of attraction that exists between hydrogen atom of one molecule and electronegative atom of another molecule is called intermolecular hydrogen atom.
H₂O

2. Intramolecular Hydrogen bond: Electrostatic force of attraction that exists between hydrogen atom of one molecule electronegative atom of the same molecule.
Example : Salicylic acid C₆H₄COOH(OH) OR Ortho-Nitrophenol

Question 23.
Balance the following redox reaction by oxidation number method.
Cr₂O^2-₇ + SO₃^2- → Cr³⁺+ SO₄^-2 + H₂O (Acid medium).
Answer:
Cr₂O^2-₇ + SO₃^2- → Cr³⁺+ SO₄^-2 (Acidic medium)
Step 1.

Multiply reduction equation by 1 unit and oxidation equation by 3 units
Cr₂O^2-₇ → 2Cr³⁺
3SO₃^2- → 3SO₄^-2
Adding both equations Cr₂O^2-₇ + 3SO₃^2- → 2Cr³⁺ + 3SO₄^2-
since there are 4 oxygen atom more on LHS, 4H₂O is added on RHS. The equation is balanced WRT H⁺ ion.
Cr₂O^2-₇ + 3SO₃^2- + 8H⁺ → 2Cr³⁺ + 3SO₄^2- + 4H₂O

Question 24.
(a) Give the cause for temporary and permanent hardness of water.
Temporary hardness

1. It is caused by bicarbonates of Ca²⁺ and Mg²⁺
2. It is removed by boiling.

Permanent hardness

1. It is caused by sulphates and chlorides of Ca²⁺ and Mg²⁺
2. It is removed by adding washing soda.

(b) Name the radioactive isotope of hydrogen.
Answer:
Tritium (₁³H) is radioactive.

Question 25.
(a) Give any two uses of sodium hydroxide.
Anwer:

1. In volumetric analysis
2. In paper industry.

(b) What is plaster of paris?
Answer:
Plaster of paris is CaSO₄ \(\frac{1}{2}\) H₂O. It is prepared by heating gypsum at 373 K.

Question 26.
(a) Which is known as inorganic benzene?
Answer:

(b) Mention the type of hybridization in diamond.
Answer:
In diamond sp³, in graphite sp².

(c) Graphite is soft and slippery. Why?
Answer:
Graphite has an hexagonal layer structure and the layers are held together by weak van der Waal’s forces. Hence, it is soft and slippery.

Part – D

IV. Answer any FIVE of the following questions carrying five marks: ( 5 × 5 = 25 )

Question 27.
(a) A compound with a molecular mass of 34 gmol^-1 is known to contain 58.8% of hydrogen and 94.12% of oxygen. Find the empirical and molecular formula of this
corn pound.
Answer:

∴ Empirical formula = H₁₀O
Molecular formula = EF × n H₁₀O × 1 = H₁₀O
But n=\(\frac{M F}{E F}=\frac{34}{26}\) = 1.3=1=

(b) Define molarity.
Answer:
It is defined as the number of moles of the solute present in 1000 ml or 1L of the solution.

(c) Express 234000 in scientific notation.
Answer:
2.3 × 10⁵

Question 28.
(a) Give any three postulates of Bohr’s atomic model of an atom.
Answer:
Bohr’s model of an atom, the postulates are:

1. Electrons revolve around the nucleus of an atom in a certain definite path called orbit or stationary state of shell.
2. The shells are having different energy levels denoted as K, L, M, N…
3. As long as the electron remains in an orbit, they neither absorb nor emit energy.
4. The electron can move only in that orbit in which angular momentum is quantized,
   i.e., the angular momentum of the electron is an integral multiple of \(\frac{h}{2 \pi}\)

(b) State Heisenberg’s uncertainty principle. Give its mathematical expression.
Answer:
It is impossible to determine both the momentum (particle nature) and position (wave nature) of a moving sub atomic particle simultaneously with absolute accuracy.
Mathematically ∆x x∆p = h / 4π where ∆x = uncertainty in position:
∆p = uncertainty in momentum; h = Plank’s constant = 6.626 × 10^-34 Js.

Question 29.
(a) Explain the significance of four quatum numbers.
Answer:
(i) Principal Quantum number : Determines energy and size of the orbital.
(n)
(ii) Azimuthal Quantum number : Determines the shape of the orbitals (sub-shells).
(l)
(iii) Magnetic Quantum number : Determines the orientation of the orbitals.
(m)
(iv) Spin Quantum number : Determines the spin motion of the orbitals.
(s)

(b) Write the electronic configuration of Chromium. (Atomic number of Chromium=24).
Answer:
Electronic configuration of chromium

Question 30.
(a) Write any three postulates of Kinetic theory of gases.
Answer:

1. Gases are made up of large number of the minute particles.
2. Pressure is exerted by a gas.
3. There is no loss of kinetic theory.
4. Molecules of gas attract on one another.
5. Kinetic energy of the molecule is directly proportional to absolute temperature.
6. Actual volume of the gaseous molecule is very small.
7. Gaseous molecules are always in motion.
8. There is more influence of gravity in the movement of gaseous molecule.

(b) Calculate the value of ‘R’ in SI units.
Answer:
In SI units, pressure is expressed in Pa and volume in m3 and 1 mole of gas at 273.15 K and 1 bar pressure occupies a volume of 22.71 L.
1 bar = 10⁵ Pa.
On substituting the values of P, V, n, T gas 1 mole of gas at STP condition in equation

Question 31.
(a) Calculate the standard enthalpy of combustion of methane. Given that standard enthalpy of combustion of carbon and hydrogen are -393.5 kJ and -285.83 kJ respectively. Standard enthalpy of formation of methane is -75.16 kJ mol^-1 .
Answer:
∆H = ∆H_fCO₂(g) + 2∆H_fH₂O(1) – ∆H_fCH₄(g) – ∆H_fO₂(g)
-890.5 kJ = -393.5 kJ + 2 × -286 kJ – ∆H_fCH₄(g) – 0; ∆H_fCH₄ = -75.0 kJ

(b) What is entropy?
Answer:
A measure of degree of disorder of a system.

Question 32.
(a) Calculate the total work done when 1 mole of a gas expands isothermally and
reversibly from an initial value of 20 dm3 to a final value of 40 dm3 at 298 K (R = 8.314 JK^-1 mol^-1 ).
Answer:
T = 27°C = 27 + 273 = 300K, V₁ = 20dm³ , V₂ = 40dm³ , R = 8.314 J/K/mol
w = – 2.303 nTlog \(\left(\frac{v_{2}}{v_{1}}\right)\)
w = -2.303 × 1 x 8.314 × 300 × log \(\left(\frac{40}{20}\right)\)
= -2.303 × 8.314 × 300 × log 2
= -2.303 × 8.314 × 300 × 0.3010
= -1729 joule = -1.723kJ.

(b) What is an extensive property? Give one example.
Answer:
It is a property which depends on the amount of the substance present in the system, e.g. mass, volume, energy.

Question 33.
(a) Calculate the PH of 0.01 M H2S04 by assuming complete ionization.
Answer:
[H⁺] = 2 × 0.01 = 0.02 = 2 × 10^-2
∴ P^H = -log₁₀ [H₂] = -log 2 × 10^-2 = 2 – 0.3010 = 1.699

(b)
Explain common ion effect with an example.
Answer:
Suppression in the degree of dissociation of a weak electrolyte by the addition of a common . ion is called common ion effect. Example: CH₃COOH and CH₃COONa.

(c) What is conjugate acid – base pair?
Answer:
Acid-base pairs which differ by one proton are called conjugate acid – base pair.
NH₃ + H₂O → NH₄⁺ + OH^–
Conjugate base of acid H₂O is OH^–; H₂ O+ H₂O → H₃O⁺ + OH^–
Conjugate acid of base NH₃ is NH₄⁺ ; NH₃ + NH₃ → NH₄⁺ + NH₂^–

Question 34.
(a) State Le Chatelier’s principle. What is the effect of pressure for the reaction?
CO(g) + 3H₂(g) ⇌ CH₄(g) + H₂O(g)
Ans. “When a constraint is applied to a system at equilibrium in a reversible reaction, the equilibrium shifts so as to nullify the constraint” [Constraint is either change in temperature or pressure or addition of reactant].
Equation shifts toward forward direction. Due to decrease in the number of moles from RHS toLHS.

(b) What is homogeneous eqilibrium? Give an example.
Answer:
A system in which different substances in equilibrium remain in a single phase is called a homogeneous equilibrium.
Example: N_2(g) + O_2(g) ≅ 2NO(g)

Part -E

V. Answer any TWO of the following questions carrying FIVE marks: ( 2 × 5 = 10 )

Question 35.
(a) Write any two difference between Inductive effect and Electromeric effect.
Inductive effect

1. It is permanent in nature.
2. It is the electronegative atom present in the molecule itself.
3. It is the mobility of electrons along C-C single bond.

Electromeric effect :

1. It is temporary in nature.
2. It is due to approach of the attaching reagent.
3. It is mobility of electrons in a multiple bond (double or triple bond).

(b) What is functional isomerism? Give an example.
Two or more compounds having same molecular formula but different functional groups are called functional isomers. Example: C₃H₆O

(c) Write bond line structural formula of (C₂H₅)₂O.
Answer:

Question 36.
(a) Describe the estimation of carbon and hydrogen by Liebig’s method.
Answer:
Principle: A known mass of an organic compound is strongly heated with dry cupric oxide (CuO), when carbon and hydrogen are quantitatively oxidized to CO₂ and H₂O respectively. The masses of CO₂ and H₂O thus formed are determined. From this, the percentages of carbon and hydrogen can be calculated.
Procedure : Pure and dry oxygen is passed through the entire assembly of the apparatus (Figure) till the CO₂ and moisture is completely removed.

A boat containing weighed organic substances is introduced inside from one end of the combustion tube by opening it for a while. The tube is now strongly heated till the whole of the organic compound is burnt up. The flow of oxygen is continued to drive CO₂ and water vapours completely to the U-tubes. The apparatus is cooled and the U-tubes are weighed separately.

Observation and calculations:

1. Mass of organic compound taken = w.g.
2. Mass of water produced = x g (Increase in mass of CaCl₂ tube).
3. Mass of carbon dioxide produced = y g (Increase in mass of KOH tube).

To determine % of carbon:
Molar mass of CO₂ = 44g mol^-1
Now, 44g of CO₂ = contains 12 g of C
y g of CO₂ will contain of \(\frac{12 y}{44}\) f of C
This amount of carbon was present in w. g. of the substance
∴ % C = \(\frac{12 y}{44} \times \frac{100}{w}\)

To determine % of Hydrogen
Molar mass of water = 18 g mol^-1
Now, 18 g of H₂O contains 2 g of H₂O
∴ x g of H₂O will contain \(\frac{2 x}{18}\) g of H₂O
This amount of hydrogen was present in weight of substance.
∴ H₂ = \(\frac{2 x}{18} \times \frac{100}{w}\)

(b) How do you detect nitrogen by using Lassaigne’s reagent?
Answer:
A few crystal of ferrous sulphate are added to the first part of the filtrate. The mixture is boiled and cooled. It is acidified with hydrochloric acid and a few drops of ferric chloride solution are added. Sodium cyanide in the filtrate reacts with ferrous to give sodium ferrocyanide. It further reacts with ferric chloride to give coloured solution of ferric ferrocyanide (Prussian blue).

Question 37.
(a) Write the mechanism of nitration of benzene.
Answer:
Nitration benzene reacts with a mixture of concentrated nitric acid and concentrated sulphuric acid at 50°C to form nitrobenzene.

Mechanism: This involves the following steps.

Step 1: Generation of electrophile nitronium ion N02
HNO₃ + 2H₂SO₄ → NO₂⁺ + H₃O⁺ + 2HSO₄^–

Step 2: The electrophile NO₂⁺ attacks the benzene ring to form a carbocation which is resonance stabilized.

Step 3: Loss of a proton to give nitrobenzene. The proton is removed by HSO₄^–

(b) Explain dehydrohalogenation reaction with an example.
Answer:
When alkyl halides are heated with alcoholic potash dehydrohalogenation takes place giving alkenes. In this reaction, halogen is removed from the α -carbon while hydrogen is removed from the β-carbon. Hence it is β-elimination reaction.