Inorganic Chemistry

Looking at the options for the composition of O₂ and N₂:
A) 29% O₂, 70% N₂
B) 19% O₂, 80% N₂
C) 21% O₂, 78% N₂
D) 23% O₂, 76% N₂

Comparing these options to the actual percentages (≈20.95% O₂ and ≈78.09% N₂), option C (21% and 78%) is the closest approximation.

In the context of the f-block of the periodic table, which is typically displayed as two rows below the main body, the lanthanoids constitute the upper row. This row consists of 14 elements, starting after Barium (Z=56) and before Hafnium (Z=72) in the main table. These 14 elements are Cerium (58), Praseodymium (59), Neodymium (60), Promethium (61), Samarium (62), Europium (63), Gadolinium (64), Terbium (65), Dysprosium (66), Holmium (67), Erbium (68), Thulium (69), Ytterbium (70), and Lutetium (71).

Given the options, 14 is present and is the standard number of elements represented in the f-block lanthanoid series, corresponding to the filling of the 4f orbitals. While the definition can sometimes include Lanthanum (making it 15), 14 is the number that fits the typical arrangement and f-orbital filling.

Let’s consider the given elements:
– F (Period 2, Group 17)
– Cl (Period 3, Group 17)
– S (Period 3, Group 16)
– P (Period 3, Group 15)

Comparing F and Cl (Group 17): Although the general trend is less negative down a group, Cl has a more negative electron gain enthalpy (-349 kJ/mol) than F (-328 kJ/mol). This is an anomaly due to the very small size of F, where the added electron experiences significant repulsion from existing electrons in the compact 2p subshell.
Comparing Cl and S (same period, different groups): Electron gain enthalpy becomes more negative across the period. Cl is in Group 17, S is in Group 16. So, Cl should have a more negative value than S. (Cl: -349 kJ/mol, S: -200 kJ/mol). This is consistent.
Comparing S and P (same period, different groups): S is in Group 16, P is in Group 15. Group 15 elements have unusually low (less negative) electron gain enthalpies due to the stable half-filled configuration. So, P should have a less negative value than S. (S: -200 kJ/mol, P: -74 kJ/mol). This is consistent.

Ranking the elements by electron gain enthalpy from most negative to least negative:
Cl (-349) > F (-328) > S (-200) > P (-74)

The element with the most negative electron gain enthalpy is Cl.
The element with the least negative electron gain enthalpy is P.

The pair is (Cl, P).

The given elements are alkali metals: Lithium (Li), Sodium (Na), Potassium (K), and Rubidium (Rb). When they form compounds, they form +1 ions: Li⁺, Na⁺, K⁺, Rb⁺.
Their ionic radii increase down the group: Li⁺ < Na⁺ < K⁺ < Rb⁺. According to Fajans' Rules, smaller cations have higher polarizing power (the ability to distort the electron cloud of the anion), which leads to increased covalent character in the bond. Li⁺ is the smallest cation among Li⁺, Na⁺, K⁺, and Rb⁺. Therefore, Li⁺ has the highest polarizing power and forms compounds with anions that exhibit the most pronounced covalent character compared to the compounds of Na, K, and Rb with the same anion. For example, LiCl has significantly more covalent character than NaCl, KCl, or RbCl.

If excess carbon dioxide is bubbled through the milky solution, the calcium carbonate precipitate reacts further to form soluble calcium bicarbonate, Ca(HCO₃)₂, and the milky appearance disappears.
CaCO₃(s) + CO₂(g) + H₂O(l) → Ca(HCO₃)₂(aq)

Other gases listed:
– SO₂ (Sulfur dioxide) also reacts with limewater, but forms calcium sulfite (CaSO₃), which is also insoluble and causes turbidity. However, the reaction with CO₂ is the most common and characteristic test for CO₂.
– NO₂ (Nitrogen dioxide) is an acidic gas and would react with Ca(OH)₂, but it typically doesn’t produce a milky precipitate with limewater as characteristically as CO₂.
– CO (Carbon monoxide) is a neutral gas and does not react with limewater.

Given the standard chemical tests, turning limewater milky is the definitive test for CO₂.