What is a mole?

• A. The mass of one mole of solute in grams.
• B. The energy content of a solution.
• C. A unit defined as the amount of substance containing as many entities as there are atoms in 12 g of carbon-12.
• D. A measure of solution temperature.

Answer: (C)

A mole is a counting unit for the amount of substance. It represents exactly 6.022 × 10^23 elementary entities (atoms, molecules, ions, etc.). This amount is tied historically to carbon-12: the number of carbon-12 atoms in 12 grams defines one mole. So one mole of any substance contains that same number of entities, regardless of what the substance is.

The option that matches this idea says the mole is defined by containing as many entities as there are in 12 g of carbon-12. That captures the fundamental concept: it's about quantity of particles, not about mass, energy, or temperature. Mass per mole (the molar mass) varies by substance—e.g., one mole of water weighs about 18.015 g, not a fixed mass for all substances—so mass or energy or temperature descriptions don’t define the mole.