Chemistry 2006 Jamb Examination

Calculation (showing steps):

One sample: 50 cm³ = 0.050 dm³ contains 34.2 g sugar.
Molar mass M = 342 g mol^-1.
Concentration g·dm^-3\[=\dfrac{34.2\ \text{g}}{0.050\ \text{dm}^3}=684\ \text{g·dm}^{-3} \] molarity (mol·dm^-3)\[=\dfrac{684\ \text{g·dm}^{-3}}{342\ \text{g·mol}^{-1}}\]\[=2.00\ \text{mol·dm}^{-3} \]

Therefore the solubility is 2.0 mol·dm^-3. This matches option A.

Cause: Acid rain is primarily caused by atmospheric oxidation of sulphur dioxide (SO₂) and nitrogen oxides (NOₓ) to form strong acids (sulphuric and nitric acids) which dissolve in rainwater.

Key reactions:
2SO₂ (g) + O₂ (g) → 2SO₃ (g)
SO₃ (g) + H₂O (l) → H₂SO₄ (aq)

These acids lower the pH of precipitation. The second paragraph: oxides of sulphur are the main contributors (option C). Other pollutants can worsen effects, but SO₂/NOₓ are the primary drivers of acid rain.

Polarity: A water molecule (H–O–H) is polar: the oxygen atom is more electronegative and pulls electron density toward itself, producing a partial negative charge at the O-end and partial positive charges on the H-ends. In effect the molecule has a positive side and a negative side.

This dipole (one positive end and one negative end) makes water an excellent solvent for ionic and polar substances because oppositely charged parts of the water molecule stabilize ions and polar molecules in solution. Therefore option B is correct.

If adding a seed crystal does not dissolve and the solution remains unchanged, the solution is at equilibrium with the solid phase — i.e. it is saturated. In a saturated solution the maximum amount of solute at that temperature is already dissolved.

In contrast, a supersaturated solution would cause the crystal to induce rapid crystallization (the crystal would grow), while an unsaturated solution would dissolve the added crystal. Thus the correct answer is saturated (D).

Count: 11 protons → atomic number 11 (sodium, Na). Neutral Na atom would have 11 electrons, but this particle has 10 electrons, so it carries a net +1 charge (Na⁺). Na⁺ is a cation — a metallic ion.

Therefore option B (metallic ion) is correct. The second paragraph explains the logic: proton number identifies the element; loss of one electron yields the common Na⁺ ion.

Chemical change produces one or more new substances with different chemical properties and compositions. For example, hydrogen and oxygen react to form water:

\[2H_2 (g) + O_2 (g) \rightarrow 2H_2O (l)\]

This reaction yields a substance (H₂O) that has properties different from the reactants.

Physical changes (melting, evaporation, dissolution without reaction) do not form new substances; they only change state or form. Therefore the hallmark difference is that a new substance is formed (option D).

At STP one mole of any ideal gas occupies 22.4 dm³. For 2.5 moles:

\[V = n \times 22.4\ \text{dm}^3 = 2.5 \times 22.4 = 56.0\ \text{dm}^3.\]

Thus the required volume is 56.0 dm³ (option D). The first paragraph shows the calculation; the second notes that molar volume at STP is the constant used.

Nuclear fission is the splitting of a heavy nucleus (e.g., U-235) into two lighter nuclei of roughly comparable mass, along with neutrons and a large amount of energy and radiation. A typical schematic:

\[^{235}\text{U} + n \rightarrow\;^{92}\text{Kr} +\;^{141}\text{Ba} + 3n + \text{energy}\]

This process releases both kinetic energy and prompt radiation; the product masses are nearly equal but not exactly identical. Option A correctly captures this description.

Molar mass M is mass per mole. Given mass = 25.0 g for n = 0.4 mol,

\[M = \dfrac{\text{mass}}{n}=\dfrac{25.0\ \text{g}}{0.4\ \text{mol}}=62.5\ \text{g·mol}^{-1}.\]

Thus the molar mass is 62.5 g·mol⁻¹ (option A). The first paragraph shows the formula; the second explains that molar mass links mass and amount of substance.

The configuration 1s² 2s² 2p³ has a total of 2+2+3 = 7 electrons, so atomic number Z = 7. The element with Z = 7 is nitrogen (N). Its ground-state configuration is exactly 1s² 2s² 2p³.

Therefore option D (nitrogen) is correct. The second sentence: count electrons → identify atomic number → match the element.