Question

Which of the following molecules contains maximum percentage of carbon by mass? 

(1) \( C _{6} H _{6} \)

(2) \( C _{2} H _{6} \)

(3) \( C _{2} H _{4} \)

(4) \( C _{3} H _{4} \)

Answer 1

C₆H₆ (Benzene)

• Molar mass of C₆H₆ = (6 × 12.01) + (6 × 1.008) = 72.06 + 6.048 = 78.108 g/mol
• Mass of carbon in C₆H₆ = 6 × 12.01 = 72.06 g
• Percentage of carbon = (72.06 / 78.108) × 100 ≈ 92.3%

C₂H₆ (Ethane)

• Molar mass of C₂H₆ = (2 × 12.01) + (6 × 1.008) = 24.02 + 6.048 = 30.068 g/mol
• Mass of carbon in C₂H₆ = 2 × 12.01 = 24.02 g
• Percentage of carbon = (24.02 / 30.068) × 100 ≈ 79.3%

C₂H₄ (Ethene)

• Molar mass of C₂H₄ = (2 × 12.01) + (4 × 1.008) = 24.02 + 4.032 = 28.052 g/mol
• Mass of carbon in C₂H₄ = 2 × 12.01 = 24.02 g
• Percentage of carbon = (24.02 / 28.052) × 100 ≈ 85.5%

C₃H₄ (Propyne)

• Molar mass of C₃H₄ = (3 × 12.01) + (4 × 1.008) = 36.03 + 4.032 = 40.062 g/mol
• Mass of carbon in C₃H₄ = 3 × 12.01 = 36.03 g
• Percentage of carbon = (36.03 / 40.062) × 100 ≈ 89.8%

Comparing the percentages:

• C₆H₆: ~92.3%
• C₂H₆: ~79.3%
• C₂H₄: ~85.5%
• C₃H₄: ~89.8%

The molecule with the maximum percentage of carbon by mass is C₆H₆ (Benzene), with approximately 92.3% carbon.