Chemistry 2017 Jamb Examination

In the contact process for manufacturing sulphuric acid, concentrated H₂SO₄ is used to absorb SO₃ gas to form oleum (H₂S₂O₇), which is later diluted with water to produce H₂SO₄. The component labelled Y is therefore concentrated H₂SO₄, as it is responsible for absorbing SO₃ gas during the process.

The relationship between the mass deposited, current, time, and Faraday’s constant is given by:\[m = \dfrac{I t M}{n F}\]where \(I = 0.5\,A\), \(t = 45\,\text{min} = 2700\,s\), \(M = 64\), \(n = 2\), and \(F = 96500\).\[m = \dfrac{0.5 \times 2700 \times 64}{2 \times 96500} = 0.448\,g\]Hence, 0.448 g of copper is deposited.

Identify the longest continuous carbon chain containing the double bond. The given structure is CH₂=C(CH₃)-CH₂-CH₃. Let's expand it: CH₃ | CH₂ = C - CH₂ - CH₃
 

Starting from the left (CH₂) and including the double bond, the chain goes through the C with the methyl group, then to the CH₂, and finally to the CH₃. This gives a chain of 4 carbons: C-C-C-C.
 

Number the carbon chain to give the double bond the lowest possible number. If we number from left to right: ¹CH₂ = ²C(CH₃) - ³CH₂ - ⁴CH₃ The double bond is between C1 and C2, so it starts at C1 (but-1-ene).
 

If we number from right to left: ⁴CH₂ = ³C(CH₃) - ²CH₂ - ¹CH₃ The double bond would be between C3 and C4, so it starts at C3 (but-3-ene).

According to IUPAC rules, we choose the numbering that gives the double bond the lower position. So, the double bond is at position 1. The parent chain is but-1-ene.
 

Identify and name any substituents. There is a methyl group (-CH₃) attached to the carbon chain. In our chosen numbering (left to right), the methyl group is on carbon 2.
 

Combine the substituent name and position with the parent chain name. The methyl group is at position 2, and the parent chain is but-1-ene. Therefore, the name is 2-methylbut-1-ene.

Rusting of iron involves oxidation of iron in the presence of oxygen and moisture, forming hydrated iron(III) oxide. The reddish-brown colour is due to Fe₂O₃.xH₂O, which is a mixture of iron(III) oxide and water molecules. Therefore, rust is best represented as Fe₂O₃.xH₂O.

According to Graham’s law of diffusion:\[\dfrac{r_X}{r_Y} = \sqrt{\dfrac{d_Y}{d_X}}\]Given \(d_X = 0.5\) and \(d_Y = 2.0\):\[\dfrac{r_X}{r_Y} = \sqrt{\dfrac{2.0}{0.5}} = \sqrt{4} = 2.0\]Hence, gas X diffuses twice as fast as gas Y.

In ethane (C₂H₆), each carbon forms four sigma (σ) bonds — three with hydrogen and one with another carbon atom. This requires sp³ hybridisation, giving a tetrahedral geometry with bond angles of approximately 109.5°.

Charles’ law states that the volume of a fixed mass of gas is directly proportional to its temperature in Kelvin at constant pressure. Extrapolating the temperature–volume graph shows that the volume becomes zero at −273°C, known as absolute zero.

Molecular mass of XO₂ = 2 × vapour density = 2 × 32 = 64
Let atomic mass of X = M.
M + 2(16) = 64
M = 64 − 32 = 32
Hence, the atomic mass of X is 32.

Gases collected by downward displacement of air must be lighter than air. Ammonia (NH₃) has a density lower than air and is highly soluble in water, so it is collected by downward displacement of air. The other gases are denser and toxic, making them unsuitable for this method.

During the preparation of trioxonitrate(V) acid (HNO₃), nitrogen(IV) oxide (NO₂) is produced as a by-product.

NaNO₃ + H₂SO₄ → NaHSO₄ + HNO₃

To remove it, air is bubbled through the acid. The oxygen in air oxidises NO₂ to NO, which then escapes, purifying the acid solution.