Arrange the following in decreasing order of their boiling points:
(I) n-Butane
(II) 2-Methylbutane,
(III) n-Pentane
(IV) 2,2-Dimethylpropane

Options

A. IgtIIgtIIIgtIV
B. IIgtIIIgtIVgtI
C. IVgtIIIgtIIgtI
D. IIIgtIIgtIVgtI

Explanation

To arrange the compounds n-Butane (I), 2-Methylbutane (II), n-Pentane (III), and 2,2-Dimethylpropane (IV) in decreasing order of their boiling points, we need to consider two main factors: the molar mass of the compounds and the degree of branching in their structures.

### Step 1: Determine the Molar Mass of Each Compound
1. **n-Butane (C4H10)**:
- Molar mass = (4 × 12) + (10 × 1) = 58 g/mol
2. **2-Methylbutane (C5H12)**:
- Molar mass = (5 × 12) + (12 × 1) = 72 g/mol
3. **n-Pentane (C5H12)**:
- Molar mass = (5 × 12) + (12 × 1) = 72 g/mol
4. **2,2-Dimethylpropane (C5H12)**:
- Molar mass = (5 × 12) + (12 × 1) = 72 g/mol

### Step 2: Analyze the Degree of Branching
- **n-Butane**: No branching, straight-chain structure.
- **2-Methylbutane**: One branch, less branched than 2,2-Dimethylpropane.
- **n-Pentane**: No branching, straight-chain structure, similar to n-Butane but with more carbons.
- **2,2-Dimethylpropane**: Highly branched, which typically leads to lower boiling points.

### Step 3: Compare Boiling Points Based on Molar Mass and Branching
- **n-Pentane (III)** has the highest boiling point due to its straight-chain structure and higher molar mass compared to n-Butane.
- **2-Methylbutane (II)** has a lower boiling point than n-Pentane but higher than n-Butane due to its branching.
- **n-Butane (I)** has a lower boiling point than both 2-Methylbutane and n-Pentane.
- **2,2-Dimethylpropane (IV)**, despite having the same molar mass as n-Pentane and 2-Methylbutane, has the lowest boiling point due to its high degree of branching.

### Final Order of Boiling Points
1. n-Pentane (III) - Highest boiling point
2. 2-Methylbutane (II)
3. n-Butane (I)
4. 2,2-Dimethylpropane (IV) - Lowest boiling point

### Conclusion
The decreasing order of boiling points is:
**n-Pentane (III) > 2-Methylbutane (II) > n-Butane (I) > 2,2-Dimethylpropane (IV)**