1.An organic compound contains C, H and S. If it contains 4% Sulphur by weight, the minimum
molecular mass of the compound is
a.200
b.400
c.600
d.800
Empirical Formula Method
Minimum molecular mass happens when sulphur content matches smallest whole number of S atoms:
- Sulphur mass fraction = 4% = 0.04
- Mass of one S atom = 32 g/mol
- Equation: MM × 0.04 = 32
- Therefore, MM = 32 / 0.04 = 800 g/mol
- The remaining 96% of the mass (768 g/mol) can be composed of carbon and hydrogen atoms feasibly.
Option Analysis
The options test your understanding of the requirement for the minimum whole-number of sulphur atoms:
- a. 200: fractional sulphur atoms (0.25), not possible
- b. 400: fractional sulphur atoms (0.50), not possible
- c. 600: fractional sulphur atoms (0.75), not possible
- d. 800: exactly 1 sulphur atom, the minimum valid molecular mass
Sulphur Atoms per Option Table
| Option | Molecular Mass (g/mol) | Sulphur Mass (4%) | Number of Sulphur Atoms | Valid? |
|---|---|---|---|---|
| a | 200 | 8 g | 0.25 | No (Fractional) |
| b | 400 | 16 g | 0.50 | No (Fractional) |
| c | 600 | 24 g | 0.75 | No (Fractional) |
| d | 800 | 32 g | 1.00 | Yes (Whole number) |
Verification Example
Example empirical formula for MM = 800:
C₆₀H₁₂₈S (60 × 12 + 128 × 1 + 32 = 720 + 128 + 32 = 800 g/mol). Sulphur percentage:
(32 / 800) × 100 = 4%, matches exactly \.
This method is a standard approach for CSIR NET Chemistry problems on percentage composition and molecular mass.
