Question

rank the elements or compounds in the table below in decreasing order of their boiling points. that is, choose 1 next to the substance with the highest boiling point, choose 2 next to the substance with the next highest boiling point, and so on.
substance | chemical symbol, formula or lewis structure | boiling point
a | h—o—o—h | select
b | hgcl₂ | 4 (lowest)
c | hcl | select
d | h₂ | select

Explanation:

To rank the substances by boiling point, we analyze the intermolecular forces (IMFs):

Step 1: Identify Substances and IMFs

• Substance A (H₂O₂): Contains hydrogen bonding (H bonded to O) and dipole-dipole, plus London dispersion forces (LDF).
• Substance B (HgCl₂): Ionic compound (Hg²⁺ and Cl⁻), so ionic bonds (strongest IMFs for these options).
• Substance C (HCl): Polar molecule, dipole-dipole forces + LDF.
• Substance D (H₂): Nonpolar, only LDF (weakest IMFs).

Step 2: Relate IMFs to Boiling Point

Boiling point depends on IMF strength:

• Ionic bonds (HgCl₂) > Hydrogen bonding (H₂O₂) > Dipole-dipole (HCl) > LDF (H₂).

Step 3: Rank in Decreasing Order

1. Highest Boiling Point: HgCl₂ (ionic, strongest IMFs) – but wait, wait! Wait, H₂O₂ has H-bonding, but HgCl₂ is ionic. Wait, no—wait, HgCl₂ is actually a covalent compound (mercury(II) chloride, molecular), but with significant polarity. Wait, correction: H₂O₂ has hydrogen bonding (strong), HgCl₂ is a polar molecule (dipole-dipole + LDF, but molar mass ~271 g/mol), HCl (molar mass ~36.5 g/mol, dipole-dipole), H₂ (molar mass 2 g/mol, LDF).

Wait, re-evaluating:

• H₂O₂: H-bonding (strong), molar mass ~34 g/mol.
• HgCl₂: Polar, molar mass ~271 g/mol (LDF increases with molar mass).
• HCl: Polar, molar mass ~36.5 g/mol.
• H₂: Nonpolar, molar mass 2 g/mol.

IMF strength hierarchy: H-bonding (H₂O₂) > Dipole-dipole (HgCl₂, HCl) + LDF (with HgCl₂ having much larger molar mass, so stronger LDF than HCl).

So correct order (decreasing boiling point):

1. H₂O₂ (A) – H-bonding + LDF (molar mass 34)
2. HgCl₂ (B) – Dipole-dipole + LDF (molar mass 271, strong LDF)
3. HCl (C) – Dipole-dipole + LDF (molar mass 36.5)
4. H₂ (D) – LDF (molar mass 2)

Wait, initial mistake: HgCl₂ is molecular (covalent), not ionic. So H-bonding in H₂O₂ is stronger than dipole-dipole in HgCl₂, but HgCl₂ has much higher molar mass, so LDF dominates? Wait, no—H-bonding is stronger than dipole-dipole + LDF for similar molar masses, but HgCl₂ is much heavier. Let’s check actual boiling points:

• H₂O₂: ~150 °C (decomposes, but boiling point ~150)
• HgCl₂: ~302 °C
• HCl: -85 °C
• H₂: -253 °C

Ah! So HgCl₂ has a higher boiling point than H₂O₂. So the correct order (decreasing boiling point) is:

1. HgCl₂ (B) – ~302 °C
2. H₂O₂ (A) – ~150 °C
3. HCl (C) – -85 °C
4. H₂ (D) – -253 °C

So the ranking is:

• Highest (1): HgCl₂ (B)
• Next (2): H₂O₂ (A)
• Next (3): HCl (C)
• Lowest (4): H₂ (D) (already marked as 4)

Answer:

1. B. HgCl₂
2. A. H₂O₂ (Lewis structure H-O-O-H)
3. C. HCl
4. D. H₂ (already set as 4)

(For the dropdowns: Highest boiling point is B (HgCl₂), next is A (H₂O₂), then C (HCl), then D (H₂).)