The chemical formula of glucose is C₆H₁₂O₆. The weight % of carbon in glucose is:

Correct — D. The stem is printed as an image in this paper. It reads: 'The chemical formula of glucose is C₆H₁₂O₆. The weight % of carbon in glucose is:'

Molar mass of C₆H₁₂O₆, adding one element at a time:
carbon 6 × 12 = 72
hydrogen 12 × 1 = 12
oxygen 6 × 16 = 96

total = 72 + 12 + 96 = 180
weight % of carbon = 72⁄180 × 100 = 40% → option (d).

Why the others are wrong

• (a)
  53 — 53 is the oxygen share, not the carbon one: 96⁄180 × 100 ≈ 53.3%. It catches anyone who divides the largest mass by the total without checking which element it belongs to.
• (b)
  25 — 25 is the share of atoms, not of mass: 6 carbon atoms out of 24 atoms in the molecule. Weight percentage needs the masses, not the head count.
• (c)
  72 — 72 is the mass of carbon in one mole, in grams. It is the numerator of the calculation, left unconverted because the division by 180 was skipped.

Percentage composition by mass = (mass of the element in one mole ÷ molar mass of the compound) × 100.

Everything turns on getting the molar mass right, so multiply each atomic mass by its subscript before adding. With C = 12, H = 1, O = 16, glucose comes to 180 g/mol, and the three fractions are 40% carbon, about 6.7% hydrogen and about 53.3% oxygen — which sum to 100.

Key facts

• The molar mass of glucose, C₆H₁₂O₆, is 180 g/mol.
• Percentage composition by mass is the element's mass in one mole divided by the molar mass, times 100.
• Carbon is 40 percent of glucose by mass, hydrogen about 6.7 percent and oxygen about 53.3 percent.
• The standard atomic masses used in these sums are C = 12, H = 1 and O = 16.

Common traps

• Stopping at 72 and reporting the mass instead of the percentage.
• Counting atoms rather than weighing them, which gives 25 instead of 40.
• Forgetting to multiply hydrogen's atomic mass by 12, which drags the total below 180.

No directly related past PYQ was found.