Chemistry 2008 Jamb Examination

 

When water is boiled, dissolved gases are expelled. The gases most readily expelled (i.e., that come out first) are those that are least soluble in water. Dissolved air is mainly nitrogen and oxygen.

Carbon(IV) oxide (CO$_2$) is significantly more soluble and tends to remain dissolved longer. Noble gases are present only in trace amounts. Consequently, the air boiled out of water (the gas released) is largely made up of the major atmospheric components N$_2$ and O$_2$. 

Sodium aluminate (NaAlO$_2$) acts as a coagulant/flocculant in water treatment. Mechanism: The coagulant neutralizes charges on colloidal and finely suspended particles, allowing them to aggregate (form flocs) and settle out in the settling tanks. Large particles settle by simple sedimentation, while the role of NaAlO$_2$ is to remove the fine (colloidal) particles that would otherwise remain suspended. 

Anhydrous cobalt(II) chloride (CoCl$_2$) is typically blue. On hydration it forms the hexaaquacobalt(II) complex [Co(H$_2$O)$_6$]$^{2+}$, which is pink. Thus the colour change on adding water to the blue anhydrous salt is blue $\rightarrow$ pink (hydrated form). $\therefore \; \text{Correct answer: B (pink).}$

Rubber (natural rubber) is non-polar and is soluble in non-polar organic solvents. Ethanol is polar and is a poor solvent for rubber. Kerosene and petrol (gasoline) are mixtures and are less commonly used in controlled laboratory preparations. Historically and industrially, aromatic solvents such as benzene (and similarly toluene, xylene) dissolve rubber readily and have been used to prepare vulcanizer's solutions or rubber solutions. Therefore, among the given choices, benzene is the appropriate solvent for preparing a rubber (vulcanizer's) solution. $\therefore \; \text{Correct answer: C (benzene).}$

\[ M_r(\text{NaCl}) = 23 + 35.5 = 58.5\;\text{g mol}^{-1} \]\[ \text{Moles of NaCl} = \dfrac{117.0}{58.5} = 2.0\;\text{mol} \]\[ \text{Volume of solution} = 1.0\;\text{dm}^3 \]\[ \therefore \; \text{Solubility} = \dfrac{2.0}{1.0} = 2.0\;\text{mol dm}^{-3} \]

Uncovered food sold alongside busy roads is exposed to vehicle emissions and dust. Lead (Pb) is commonly deposited from traffic exhaust and contaminated dust, so such food is likely to contain traces of Pb.

\[ [\text{OH}^-] = 0.001 = 1.0 \times 10^{-3}\;\text{mol dm}^{-3} \]\[ \text{pOH} = -\log_{10}(1.0\times10^{-3}) = 3 \]\[ \text{pH} = 14 - 3 = 11 \]

Basicity is the number of replaceable hydrogen ions in an acid molecule. Acetic acid (CH₃COOH) has only one replaceable hydrogen in the \(-COOH\) group, so its basicity is 1.

Reaction: HCl + NaOH → NaCl + H₂O
C_aV_a = C_bV_b 
C_a = 0.05, V_a = 32.00 cm³ = 0.03200 dm³
Moles of HCl = 0.05 × 0.03200 = 0.0016 mol
Moles of NaOH = 0.0016 mol
V_b = 25 cm³ = 0.025 dm³
 \[ C_b = \dfrac{0.0016}{0.025} = 0.064\;\text{mol dm}^{-3} \]