On the basis of molecular structure and bond polarity, which of the following compounds is most like — Solutions Chemistry Question

STEPS:

1. Recall the central rule of solubility: The solubility of a solute in a solvent is governed by the principle of "like dissolves like." Polar solutes dissolve well in polar solvents, while nonpolar solutes dissolve in nonpolar solvents.
2. Identify the nature of the solvent (water): Water ($\text{H}_2\text{O}$) is a highly polar solvent. Its molecules are held together by a strong network of hydrogen bonds, which are exceptionally strong dipole-dipole attractions.
3. Determine the requirements for high solubility in water: For a substance to dissolve significantly in water, the new solute-solvent attractions must be strong enough to overcome the solute-solute and solvent-solvent attractions. The most soluble substances are those that are polar and, ideally, capable of forming hydrogen bonds directly with water molecules.
4. Analyze the molecular structure and polarity of each option:
* $\text{CH}_4$ (Methane): Has a symmetrical tetrahedral geometry. The $\text{C–H}$ bonds are essentially nonpolar, and the molecular symmetry cancels out any minor dipoles, making methane completely nonpolar.
* $\text{CCl}_4$ (Carbon tetrachloride): Has a symmetrical tetrahedral geometry. Even though the individual $\text{C–Cl}$ bonds are polar, their dipoles pull equally in opposite directions and cancel out completely, making the overall molecule completely nonpolar.
* $\text{NH}_3$ (Ammonia): Has an asymmetric trigonal pyramidal geometry with a lone pair on the highly electronegative nitrogen atom. This asymmetry and the large electronegativity difference between nitrogen and hydrogen make it highly polar. Crucially, the presence of $\text{N–H}$ bonds allows it to form strong hydrogen bonds with water.
* $\text{PH}_3$ (Phosphine): Has a trigonal pyramidal molecular geometry. However, phosphorus and hydrogen have nearly identical electronegativities ($\text{P} = 2.19$, $\text{H} = 2.20$), making the $\text{P–H}$ bonds nonpolar. Additionally, phosphorus is not highly electronegative enough (unlike $\text{N}$, $\text{O}$, or $\text{F}$) to participate in hydrogen bonding.
5. Compare the solute-solvent interactions: Ammonia ($\text{NH}_3$) is the only substance among the choices that is both highly polar and capable of forming strong hydrogen bonds with water molecules. Consequently, it integrates into water's hydrogen-bonding network easily, giving it the greatest solubility. This confirms Option C as the correct answer.

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WHY_OTHERS_WRONG:

• Option A is incorrect: Methane ($\text{CH}_4$) is nonpolar and can only interact with water through extremely weak dipole-induced dipole forces. These weak forces cannot disrupt the strong hydrogen bonds holding water molecules together, making methane practically insoluble in water.
• Option B is incorrect: Carbon tetrachloride ($\text{CCl}_4$) is a large, nonpolar molecule. It cannot form hydrogen bonds with water, and the weak dispersion forces it offers are insufficient to allow it to dissolve in a polar solvent like water.
• Option D is incorrect: Although phosphine ($\text{PH}_3$) has a similar trigonal pyramidal shape to ammonia, the $\text{P–H}$ bonds are nonpolar due to the nearly equal electronegativities of phosphorus and hydrogen. Because it is nonpolar and cannot participate in hydrogen bonding with water, it has a much lower solubility than ammonia.