Comparing carbon content in hydrocarbons

Steps:

• Recall formulas: alkanes are CnH2n+2, alkenes are CnH2n.
• For same n, alkenes have fewer H atoms, so higher carbon mass percentage.
• Evaluate A: unsaturation doesn't reduce solubility; both types are nonpolar and insoluble in water.
• Evaluate C and D: cracking yields mixed products; bromine adds as 1 mol Br2 (160 g) per mol alkene.

Why B is correct:

• Alkenes (CnH2n) have two fewer H atoms than alkanes (CnH2n+2) for equal carbons, increasing %C by mass (e.g., ethene 85.7% vs. ethane 80%).

Why the others are wrong:

• A: Solubility in water is low for both due to nonpolarity; unsaturation has minimal impact.
• C: Cracking large alkanes yields a mixture of smaller alkanes, alkenes, and hydrogen, not only smaller carbon-hydrogen compounds.
• D: One mole of alkene adds one mole of Br2 (160 g), not 80 g.

Final answer: B