Chemistry 2009 Jamb Examination

$$ M(\mathrm{Pb(NO_3)_2}) $$ $$= 207 + 2\big(14 + 3\times16\big) $$ $$ = 207 + 2(62) = 207 + 124 $$ $$ = 331\ \text{g mol}^{-1} $$

$$ n = \dfrac{10.5\ \text{g}}{331\ \text{g mol}^{-1}} = 0.0317\ \text{mol} $$

$$ 20\ \text{cm}^3 = 0.020\ \text{dm}^3 $$

$$ c = \dfrac{n}{V} = \dfrac{0.0317}{0.020} = 1.59\ \text{mol dm}^{-3} $$

Solubility depends strongly on the nature of the solvent because solute–solvent interactions (polarity, hydrogen bonding, dispersion forces) differ between solvents. Even at identical temperatures, a solute will dissolve to different extents in, for example, water versus ethanol.

Therefore the concentration of a saturated solution of a particular solute is generally different in different solvents at the same temperature — option B — because the equilibrium between dissolved and undissolved solute shifts with solvent properties.

Oxides of nitrogen (NO and NO$_2$, collectively NOx) form when atmospheric N$_2$ reacts with O$_2$ at high combustion temperatures. Modern major sources include internal-combustion engines and industrial furnaces burning fossil fuels (petrol, diesel, natural gas), so 'fuel' is the broad, correct choice.

While coal and wood also produce NOx when burned, the term 'fuel' (especially vehicle and industrial fossil fuels) best captures the primary contemporary source of NOx emissions.

The notation $^{A}_{Z}X$ gives mass number $A$ (total protons + neutrons) and atomic number $Z$ (protons). Here $Z=11$ and $A=23$, so the element has 11 protons and 12 neutrons, which corresponds to sodium ($^{23}Na$).

Sodium (Z = 11) is a group 1 element (an alkali metal). Therefore the correct deduction is that the atom is an alkali metal (option D). The atomic number is not 23 — 23 is the mass number in this case.

The principal quantum number $n$ identifies the main shell (energy level). The azimuthal (angular momentum) quantum number, usually written $\ell$, divides each shell into subshells or orbital types (s, p, d, f, etc.), specifying the shape of the electron cloud.

The magnetic quantum number $m_{\ell}$ then specifies orientation of an orbital within a subshell, and spin $m_s$ is the intrinsic spin of the electron. Thus the azimuthal quantum number is the correct answer (option B).

In the complex ion $[Cu(NH3)4]^{2+}$, each ammonia ligand donates a lone pair to the central Cu$^{2+}$ ion to form dative (coordinate) covalent bonds. This is the hallmark of coordination chemistry: ligand → metal electron-pair donation.

Although the bonds have covalent character, the correct descriptive term for these donor–acceptor interactions in a complex is 'coordinate' bonding (option A).

Photographic flash tubes commonly use mixtures of heavy noble gases to produce intense, bright flashes when electrically discharged. Xenon is especially prized for producing bright, daylight-like flashes; krypton is often blended to alter discharge properties and cost. The typical, widely-cited pair used in flash tubes is krypton and xenon.

Therefore option B (krypton and xenon) is the correct choice for the gas mixture used in photographic flash applications.

Freezing ice cream (liquid → solid) is a phase change where the chemical identity of the constituents remains the same; it is thus a physical change and generally reversible by melting. Physical changes alter state or form but not molecular identity.

The other processes listed (dissolving reactive metals like calcium which may involve chemical reaction; burning kerosene—combustion; exposure of white phosphorus to air—oxidation) are chemical changes, so freezing ice cream (option A) is the physical change.

$$ \begin{aligned} M(ZnSO_4) &= 65 + 32 + 4(16) = 161\ \text{g mol}^{-1} \\[6pt] M(7H_2O) &= 7 \times 18 = 126\ \text{g mol}^{-1} \\[6pt] M(ZnSO_4 \cdot 7H_2O) &= 161 + 126 = 287\ \text{g mol}^{-1} \\[6pt] \text{Mass fraction of water} &= \dfrac{126}{287} = 0.4389 \\[6pt] \%\ \text{of water} &= 0.4389 \times 100 = 43.9\% \approx 44.0\% \end{aligned} $$ $$ \boxed{44.0\%\ \text{(Option B)}} $$

$$ CaCO_3 + 2HCl \to CaCl_2 + H_2O + CO_2$$

$$2 \times 0.0075 = 0.015\ \text{mol}$$

$$0.0075 \times 22.4 = 0.168\ \text{dm}^3 = 168\ \text{cm}^3$$