In which of the following organic compounds is the hydrogen-oxygen ratio equal to 2:1?

Analyze the question: It asks which organic compound has hydrogen and oxygen in the ratio 2:1, the same ratio as in water (H₂O).

Explain the concept: Carbohydrates are made of carbon, hydrogen and oxygen. Their general formula is Cₓ(H₂O)ᵧ. The name "carbo-hydrate" means "hydrated carbon", so the hydrogen and oxygen are in the same 2:1 ratio as in water.

Step-by-step solution:

1. Glucose: C₆H₁₂O₆. The H:O ratio is 12:6 = 2:1 ✔
2. Sucrose: C₁₂H₂₂O₁₁. The H:O ratio is 22:11 = 2:1 ✔
3. Starch: (C₆H₁₀O₅)ₙ. The H:O ratio is 10:5 = 2:1 ✔

Now check the other options:

• Proteins contain C, H, O, N and often S. The ratio is not 2:1.
• Lipids (fats and oils) have very little oxygen compared with hydrogen. For example, tristearin is C₅₇H₁₁₀O₆, so H:O is about 18:1.
• Vitamins are a varied group with no fixed ratio.

Answer: B. Carbohydrates

Tip: Remember "Carbo-HYDRATE = water ratio", meaning H₂O and so 2:1. Lipids are the opposite: they have very little oxygen, which is why they store more energy per gram.