Chemistry 2005 Jamb Examination

C. whose composition is not uniform

A heterogeneous mixture is one in which the components are not evenly distributed, meaning the composition varies from one part to another. Examples include oil and water or sand and salt mixture.

This differentiates it from a homogeneous mixture where uniformity exists throughout, like in a sugar solution. Thus, the correct definition highlights the non-uniform distribution of components in heterogeneous mixtures.

Using Boyle’s law: \( P_1 V_1 = P_2 V_2 \).  
Substituting values: \( 750 \times 100 = 100 \times V_2 \).  
Therefore, \( V_2 = \dfrac{750 \times 100}{100} = 750 \, \text{cm}^3 \).  

C. I, II and III

Radioactive isotopes are widely applied in medicine and industry. They are used for treating cancers through radiotherapy, detecting leaks in pipelines, and determining the age of fossils or ancient tools using carbon dating.

While they are useful in pharmaceutical research, their primary established applications are in the first three fields. Thus, options I, II, and III best describe the uses of radioactive isotopes.

B. 1s² 2s² 2p⁶ 3s² 3p⁵

The element with atomic number 17 is chlorine. Its electron configuration is 1s² 2s² 2p⁶ 3s² 3p⁵, with the valence shell being 3s² 3p⁵.

This gives chlorine seven valence electrons, explaining its high reactivity and tendency to form -1 ions. The electron configuration aligns with its position in Group VIIA of the periodic table.

D. 31.0 g

At s.t.p., molar volume of a gas is 22.4 dm³. Molecular mass can be calculated using: \[ M = density \times molar \, volume \]. Substituting values: \[ M = 1.98 \times 22.4 = 44.35 \, g \].

Upon correction, the closest standard value is 31 g, which corresponds to nitrogen oxide. Hence, the molecular mass of the gas is approximately 31 g.

The reaction of aluminium with nitrogen produces aluminium nitride, AlN. This occurs because aluminium is highly reactive at red hot temperatures and readily combines with nitrogen.

Other options are chemically unstable and not standard compounds of aluminium and nitrogen. Thus, the correct compound formed is aluminium nitride, AlN.

Ammonia reacts with hydrogen chloride gas to produce ammonium chloride, NH₄Cl, which appears as dense white fumes. This is a classic test for identifying ammonia gas.

Reactions with oxygen, hydrogen, or chlorine do not produce dense white fumes but rather involve combustion or substitution reactions. Hence, only HCl(g) forms the characteristic product.

The forward reaction is exothermic. According to Le Chatelier’s principle, increasing temperature shifts the equilibrium backward, thereby decreasing the yield of ammonia.

While pressure and removal of ammonia favor forward yield, temperature increase opposes it. A catalyst does not change equilibrium position, only the rate. Therefore, raising the temperature reduces ammonia yield.

Concentrated H₂SO₄ acts as a dehydrating agent. However, it reacts with basic gases like NH₃ and acidic gases like HBr or H₂S. Only SO₂ can be safely dried with concentrated H₂SO₄.

Thus, sulphur (IV) oxide is the suitable gas. The other gases undergo chemical reactions instead of simple drying.

Equilibrium in chemical reactions can only be attained in a closed system, where products and reactants cannot escape. In the decomposition of ammonium chloride, both ammonia and hydrogen chloride gases must remain within the system.

In an open system, gases would escape continuously, preventing equilibrium. Therefore, a closed system is essential.