Which of the following substances contains the most usable energy per kilogram?

• A. Methane (CH_4)
• B. Ethanol (C_2H_5OH)
• C. Hydrogen gas (H_2)
• D. Coal (C)

Answer: (C)

Verified step by step guidance

1. Understand that 'usable energy per kilogram' refers to the energy content or energy density of a fuel, often measured as the amount of energy released during combustion per unit mass.
2. Identify that the substances listed are different fuels: Methane (CH_4), Ethanol (C_2H_5OH), Hydrogen gas (H_2), and Coal (C), each with distinct chemical compositions and energy contents.
3. Recall that the energy released during combustion depends on the chemical bonds broken and formed; hydrogen gas (H_2) has a very high energy content per unit mass because it is the lightest element and releases a lot of energy when it reacts with oxygen to form water.
4. Compare the typical energy densities (in MJ/kg) of each substance: Methane, Ethanol, Coal, and Hydrogen gas, noting that hydrogen has the highest energy per kilogram despite its low density as a gas.
5. Conclude that hydrogen gas contains the most usable energy per kilogram because it has the highest energy density by mass among the options given.