Chemistry 2015 Jamb Examination

The molecular formula C₂H₆O corresponds to compounds having two carbon atoms, six hydrogen atoms, and one oxygen atom. Ethanol (C₂H₅OH) fits this composition. Another compound with this formula is dimethyl ether, but among the given options, only ethanol matches it.

Sodium hydroxide is a strong base and completely dissociates in water:\[ \text{NaOH} \rightarrow \text{Na}^+ + \text{OH}^- \] Therefore, \([OH^-] = 0.001 = 10^{-3}\).\[ pOH = -\log [OH^-] \]\[ = -\log(10^{-3}) = 3 \]\[ pH + pOH = 14 \]\[ \Rightarrow pH = 14 - 3 = 11 \]Thus, the pH of the solution is 11.

The reaction involves one mole of dinitrogen tetroxide producing two moles of nitrogen dioxide. Because the reaction is endothermic, increasing temperature shifts the equilibrium to the right. Similarly, decreasing pressure favours the side with more gas molecules. Hence, a decrease in pressure favours more formation of NO₂.

Chlorine is a strong oxidising agent and can oxidise potassium iodide on damp starch-iodide paper to iodine. The liberated iodine reacts with the starch to form a deep blue complex. This reaction is often used as a test for the presence of chlorine gas.

Let the percentage abundance of \(^{35}Cl\) be \(x\) and \(^{37}Cl\) be \(100 - x\).
The average atomic mass is given by:\[35x + 37(100 - x) = 35.5 \times 100\]\[3500 + 200x = 3550\]\[200x = 50\]\[x = 0.25 = 25\%\]Therefore, \(^{37}Cl\) has a relative abundance of 25%. The slight difference in isotope masses gives the weighted average of 35.5 observed in chlorine’s atomic mass.

At the anode (positive electrode), oxidation occurs. In the electrolysis of aqueous CuSO₄ using platinum electrodes, water is oxidised to form oxygen gas according to the reaction:\[4OH^- \rightarrow 2H_2O + O_2 + 4e^-\]Hence, oxygen is liberated at the anode while copper is deposited at the cathode.

Let the oxidation state of sulphur be \(x\). For H₂SO₄:
Hydrogen contributes \(+1 \times 2 = +2\) and oxygen contributes \(-2 \times 4 = -8\). The sum of oxidation states equals zero for a neutral compound:\[2 + x - 8 = 0\]\[x = +6\]Hence, the oxidation state of sulphur in H₂SO₄ is +6.

During the electrolysis of brine (concentrated NaCl solution), a carbon or graphite anode is typically used because it is inert and does not react with chlorine gas produced during the process. Platinum can also be used but is more expensive, making carbon the preferred industrial choice.

The relative molecular mass (Mr) is the sum of the atomic masses of all atoms present in the molecule.\[\text{Mr} = 40 + 12 + (16 \times 3)\]\[\text{Mr} = 40 + 12 + 48\]\[\text{Mr} = 100\]Therefore, the relative molecular mass of CaCO₃ is 100.

When steam is passed over red hot carbon, a redox reaction occurs in which water vapour is reduced to hydrogen gas and carbon is oxidised to carbon monoxide. The reaction is known as the water–gas reaction and is represented by:\[C + H_2O \rightarrow CO + H_2\]This mixture of CO and H₂ is known as water gas and is used as a fuel.