Chemistry · Sample Paper 3

General instructions

Section A · Objective

1. Question 11 mark

   The number of moles of oxygen atoms in 1 mol of $CaC{O}_3$ is

   1. A.1
   2. B.2
   3. C.3
   4. D.5
2. Question 21 mark

   The molarity of a solution with 0.5 mol NaCl in 250 mL of solution is

   1. A.0.5 M
   2. B.1 M
   3. C.2 M
   4. D.0.125 M
3. Question 31 mark

   The number of orbitals in the 3d subshell is

   1. A.3
   2. B.5
   3. C.7
   4. D.1
4. Question 41 mark

   According to Aufbau principle, which orbital is filled first?

   1. A.3d
   2. B.4s
   3. C.4p
   4. D.4d
5. Question 51 mark

   Elements of group 18 are called

   1. A.alkali metals
   2. B.halogens
   3. C.noble gases
   4. D.chalcogens
6. Question 61 mark

   Which of these is the most metallic element?

   1. A.Na
   2. B.Mg
   3. C.Al
   4. D.Si
7. Question 71 mark

   The hybridisation of carbon in methane is

   1. A.sp
   2. B.sp²
   3. C.sp³
   4. D.dsp²
8. Question 81 mark

   The number of σ and π bonds in ethene ($C_2{H}_4$) is

   1. A.5σ, 1π
   2. B.4σ, 2π
   3. C.6σ, 0π
   4. D.3σ, 3π
9. Question 91 mark

   In an isolated system,

   1. A.energy and matter are exchanged
   2. B.only energy is exchanged
   3. C.neither energy nor matter is exchanged
   4. D.only matter is exchanged
10. Question 101 mark

    The pH of 0.001 M NaOH is

    1. A.3
    2. B.11
    3. C.7
    4. D.13
11. Question 111 mark

    The oxidation number of S in $H_{2}S{O}_4$ is

    1. A.+4
    2. B.+6
    3. C.−2
    4. D.+2
12. Question 121 mark

    The IUPAC name of $C{H}_{3}COC{H}_3$ is

    1. A.propanal
    2. B.propanone
    3. C.ethanone
    4. D.acetone acid
13. Question 131 mark

    Assertion (A): Noble gases have very high ionisation enthalpies.

    Reason (R): Noble gases have a stable closed-shell configuration.

    1. A.Both Assertion (A) and Reason (R) are true, and R is the correct explanation of A.
    2. B.Both Assertion (A) and Reason (R) are true, but R is not the correct explanation of A.
    3. C.Assertion (A) is true, but Reason (R) is false.
    4. D.Assertion (A) is false, but Reason (R) is true.
14. Question 141 mark

    Assertion (A): $H_{2}O$ has a bent shape.

    Reason (R): The oxygen atom has two lone pairs that repel the bond pairs.

    1. A.Both Assertion (A) and Reason (R) are true, and R is the correct explanation of A.
    2. B.Both Assertion (A) and Reason (R) are true, but R is not the correct explanation of A.
    3. C.Assertion (A) is true, but Reason (R) is false.
    4. D.Assertion (A) is false, but Reason (R) is true.
15. Question 151 mark

    Assertion (A): A buffer resists changes in pH on adding small amounts of acid or base.

    Reason (R): A buffer contains a weak acid and its conjugate base (or a weak base and its conjugate acid).

    1. A.Both Assertion (A) and Reason (R) are true, and R is the correct explanation of A.
    2. B.Both Assertion (A) and Reason (R) are true, but R is not the correct explanation of A.
    3. C.Assertion (A) is true, but Reason (R) is false.
    4. D.Assertion (A) is false, but Reason (R) is true.
16. Question 161 mark

    Assertion (A): Benzene is highly unsaturated yet does not readily undergo addition reactions.

    Reason (R): The π electrons in benzene are delocalised, giving it extra stability (aromaticity).

    1. A.Both Assertion (A) and Reason (R) are true, and R is the correct explanation of A.
    2. B.Both Assertion (A) and Reason (R) are true, but R is not the correct explanation of A.
    3. C.Assertion (A) is true, but Reason (R) is false.
    4. D.Assertion (A) is false, but Reason (R) is true.

Section B · Very short answer

1. Question 172 marks

   Calculate the number of atoms in 8 g of oxygen gas ($O_2$).

2. Question 182 marks

   State Pauli's exclusion principle and Hund's rule.

3. Question 192 marks

   Why is the atomic radius of a noble gas usually larger than that of the halogen in the same period?

4. OR

   Question 192 marks

   Write the general electronic configuration of s-block and p-block elements.

5. Question 202 marks

   For the reaction $2A(g)+B(g)\to 2D(g)$, $\Delta U^{\circ }=-10.5$ kJ and $\Delta S^{\circ }=-44.1{\text{J K}}^{-1}$. Calculate $\Delta G^{\circ }$ at 298 K (take $\Delta H\approx \Delta U+\Delta n_{g}RT$).

6. Question 212 marks

   Write the structures of the cis and trans isomers of but-2-ene. Which has a higher dipole moment?

Section C · Short answer

1. Question 223 marks

   Calculate the mass of $CaO$ produced by heating 50 g of $CaC{O}_3$. Also find the volume of $C{O}_2$ at STP (22.4 L/mol).

2. Question 233 marks

   What is the photoelectric effect? The threshold frequency of a metal is $7\times 10^{14}$ Hz. Find the kinetic energy of an electron emitted when light of frequency $1\times 10^{15}$ Hz falls on it. ($h=6.6\times 10^{-34}$ J s)

3. Question 243 marks

   Draw (describe) the MO diagram of O₂. Find its bond order and explain why it is paramagnetic.

4. Question 253 marks

   State Hess's law. Calculate the enthalpy of formation of CO from: C + O₂ → CO₂, ΔH = −393.5 kJ; CO + ½O₂ → CO₂, ΔH = −283 kJ.

5. Question 263 marks

   For $PC{l}_5\rightleftharpoons PC{l}_3+C{l}_2$, the equilibrium concentrations are $[PC{l}_5]=0.1$, $[PC{l}_3]=0.2$, $[C{l}_2]=0.2$ mol/L. Calculate $K_c$. What happens if the volume is decreased?

6. OR

   Question 263 marks

   What is the common ion effect? Explain with an example.

7. Question 273 marks

   Balance by the oxidation number method: $C{r}_2{O}_7^{2-}+S{O}_2\to C{r}^{3+}+S{O}_4^{2-}$ (acidic medium).

8. Question 283 marks

   Explain the resonance effect with an example. How does it differ from the inductive effect?

Section D · Case-based

1. Question 294 marks

   Case study — Hand warmers. Some hand warmers contain iron powder that rusts quickly when the packet is opened: $4Fe(s)+3O_2(g)\to 2F{e}_2{O}_3(s)$, $\Delta H=-1648$ kJ. The heat released keeps the hands warm for hours.

   (i) Is this reaction exothermic or endothermic? (1 mark)

   (ii) What is the sign of $\Delta S$ for this reaction? Why? (1 mark)

   (iii) Use $\Delta G=\Delta H-T\Delta S$ to explain why the reaction is spontaneous at room temperature. (2 marks)

   OR

   (iii) How much heat is released when 11.2 g of iron rusts? (Fe = 56) (2 marks)

2. Question 304 marks

   Case study — Plastic from ethene. Ethene, obtained by cracking petroleum, is the starting material for polythene. It also decolourises bromine water and reacts with hydrogen in the presence of nickel. Ethyne, another hydrocarbon, is used in welding torches.

   (i) Write the IUPAC name and formula of ethyne. (1 mark)

   (ii) What does the decolourisation of bromine water show? (1 mark)

   (iii) Write the reactions of ethene with (a) hydrogen (b) bromine. (2 marks)

   OR

   (iii) How is ethyne prepared from calcium carbide? Why is it used in welding? (2 marks)

Section E · Long answer

1. Question 315 marks

   (a) What are quantum numbers? Explain the significance of n, l, $m_l$ and $m_s$. (b) Write the electronic configurations of Cu (Z = 29) and Fe³⁺ (Z = 26).

2. OR

   Question 315 marks

   Explain the periodic trends in (a) electronegativity (b) metallic character (c) electron gain enthalpy, giving reasons and examples.

3. Question 325 marks

   (a) Explain Le Chatelier's principle with reference to the Haber process. (b) Calculate the pH of a solution obtained by mixing 50 mL of 0.1 M HCl with 50 mL of 0.05 M NaOH.

4. Question 335 marks

   (a) Describe the principle and use of chromatography. (b) How are nitrogen, sulphur and halogens detected by Lassaigne's test? (c) What is the Kjeldahl method used for?