CDS (I) 2022 General Knowledge · Question 7 of 120

Correct — C, 6. Complete combustion means every carbon ends up as carbon dioxide and every hydrogen as water. Glucose is C₆H₁₂O₆, so one mole of it must yield six moles of CO₂ and six moles of H₂O. Counting oxygen atoms settles the rest: the products between them hold 6 × 2 + 6 = 18 oxygen atoms, and the glucose molecule brings only 6 of those with it, so 12 oxygen atoms have to come from the air. Twelve atoms is six molecules, and the balanced equation is C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O. That same equation, run slowly and in stages inside a cell, is aerobic respiration, which is why the number six turns up again in biology.

• (a)1 — Treats the reaction as though a single oxygen molecule could serve six carbons. One O₂ supplies only two oxygen atoms; six carbons alone need twelve just to become carbon dioxide.
• (b)3 — Half the requirement. Three moles of O₂ carry six oxygen atoms, enough to convert only three of the six carbons, so the equation would not balance.
• (d)12 — The number of oxygen ATOMS drawn from the air, offered as if it were the number of molecules. Oxygen gas is diatomic, so twelve atoms are six molecules — this is the sharpest trap in the item, and the one to watch for.

Balancing a combustion equation is a bookkeeping exercise in three steps. Fix the carbons first, because each becomes one CO₂; fix the hydrogens next, because each pair becomes one H₂O; then count the oxygen atoms on the product side, subtract whatever the fuel itself supplied, and divide the remainder by two to convert atoms into molecules of O₂. For glucose that gives 6 CO₂, 6 H₂O, 18 oxygen atoms needed, 6 supplied by the sugar, 12 drawn from the air and therefore 6 molecules of oxygen. The same equation describes aerobic respiration, in which the energy is released gradually through glycolysis, the Krebs cycle and the electron transport chain rather than all at once as heat.

The item can be answered without writing anything down if the balanced equation for respiration is already familiar, because glucose and oxygen appear there in a one-to-six ratio. If it is not, the atom count takes about thirty seconds and is safer than recall. The one place candidates lose the mark is at the final division: the arithmetic naturally produces twelve, which is a count of atoms, and the question asks for moles of oxygen GAS, which is diatomic. Halving twelve to get six is the step the fourth option is waiting to catch.

• Glucose has the molecular formula C₆H₁₂O₆.
• The balanced equation for its complete combustion is C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O.
• Six carbons demand six CO₂ and twelve hydrogens demand six H₂O, giving eighteen oxygen atoms in the products.
• Glucose itself supplies six of those oxygen atoms, so twelve — that is six O₂ molecules — must come from outside.
• The same overall equation describes aerobic respiration, releasing about 2870 kJ per mole of glucose.

• Reporting the number of oxygen atoms instead of the number of oxygen molecules.
• Forgetting that the fuel itself already contains oxygen, which reduces how much air is needed.
• Assuming the water produced supplies its own oxygen from somewhere other than the reactants.

As a stoichiometric count like this one, as a balancing exercise on a given equation, or through the biology side as the overall equation of respiration.

• practice — not a real PYQ

  The complete combustion of one mole of methane, CH₄, requires how many moles of oxygen gas?

  • (a)1
  • (b)2
  • (c)3
  • (d)4

  Answer(b) 2 — the balanced equation is CH₄ + 2O₂ → CO₂ + 2H₂O, needing four oxygen atoms and therefore two diatomic molecules.

• practice — not a real PYQ

  In the complete combustion of one mole of glucose, the number of moles of carbon dioxide produced is

  • (a)3
  • (b)6
  • (c)12
  • (d)18

  Answer(b) 6 — glucose contains six carbon atoms, and each one leaves as a single molecule of carbon dioxide.