Chemistry 2007 Jamb Examination
Question 21
Number of moles (n) is given by:
\[ n = C \times V \]
Where C = concentration in mol dm⁻³, V = volume in dm³.
Convert 20 cm³ to dm³: \( 20/1000 = 0.020\,\text{dm³} \).
Substitute values: \( n = 0.75 \times 0.020 = 0.015\,\text{mol} \).
Hence, the number of moles of HCl is 0.015 mol (option A).
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Question 22
Tetraoxosulphate (VI) acid = H₂SO₄.
Let the oxidation state of S be x, O is usually -2. Total charge = 0:
\[ 2(+1) + x + 4(-2) = 0 \]
\[ 2 + x - 8 = 0 \]
\[ x = +6 \]
Thus, the oxidation state of oxygen = -2 (option B).
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Question 23
MnO₂(s) + x HCl(aq) → MnCl₂(aq) + y H₂O(l) + z Cl₂(g). Determine the values of x, y, z respectively
MnO2 + 4HCl → MnCl2 + 2H2O + Cl2
Count the molecules:
x = 4 (HCl)
y = 2 (H2O)
z = 1 (Cl2)
MnO2 + 4HCl → MnCl2 + 2H2O + Cl2
Therefore: x = 4, y = 2, z = 1
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Question 24
Calculate the mass of chlorine gas which occupies a volume of 1.12 dm³ at s.t.p [Cl = 35.5, molar volume = 22.4 dm³]
Calculate moles using molar volume:
\[ n = V/V_m = 1.12/22.4 = 0.05\,\text{mol} \]
Molar mass of Cl₂ = 2 × 35.5 = 71 g/mol
Mass = n × M = 0.05 × 71 = 3.55 g
Oops! Option C = 3.55 g. Correct answer is 3.55 g (option C).
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Question 25
The Millikan oil-drop experiment measured the charge of a single electron by observing the motion of charged oil droplets in an electric field. Option B is correct.
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Question 26
What is the volume of oxygen required to burn completely 45 cm³ of methane at s.t.p?
The balanced combustion reaction of methane (CH₄) is:
\[ CH_4 + 2O_2 → CO_2 + 2H_2O \]
This shows that 1 volume of CH₄ reacts with 2 volumes of O₂.
Given volume of methane = 45 cm³
\[ V_{O_2} = 2 imes 45 = 90 \text{ cm³} \]
Correction: Actually option D = 90.0 cm³. Hence the oxygen volume required is 90 cm³ (option D).
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Question 27
Atomic number of phosphorus = 15
Electron configuration: 1s² 2s² 2p⁶ 3s² 3p³
L-shell = 2nd shell = 2s² + 2p⁶ = 8 electrons
Hence, number of electrons in the L-shell = 8 (option B).
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Question 28
If the electron configuration of an element is 1S2 2S2 2P5, how many unpaired electrons are there?
Electron configuration: 1s² 2s² 2p⁵
2p orbital has 3 sub-orbitals: ↑↓, ↑, ↑ (one unpaired electron)
Hence, there is 1 unpaired electron (option C).
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Question 29
Compressing a gas reduces its volume.
Average intermolecular distance \( r \) is proportional to \( V^{1/3} \). Halving volume decreases \( r \).
Other quantities:
- Atomic radius does not change.
- Number of molecules remains constant.
- Frequency of collisions increases (does not decrease).
Hence, the average intermolecular distance decreases (option B).
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Question 30
The third period of the periodic table contains elements with principal quantum number n=3: Na, Mg, Al, Si, P, S, Cl, Ar.
Option D includes: Na, Mg, S, Ar, which are all in the third period.
Hence correct answer is option D.