Avogadro’s number definition:\nAvogadro number is the number of molecules contained in one __________ of a gas (i.e., one chemical mole).

Introduction / Context:
Avogadro’s number NA is a cornerstone of chemistry and physics, linking macroscopic amounts to microscopic counts. It specifies how many discrete particles (molecules, atoms, ions) are present in exactly one mole of a substance, enabling conversions between mass, amount, and number of entities.

Given Data / Assumptions:

• One mole is the amount of substance containing exactly NA entities.
• For gases, one mole occupies different volumes depending on T and P; the count does not change.
• We seek the correct fill-in describing the basis amount.

Concept / Approach:
By definition, Avogadro’s number is the number of specified particles in one mole, historically called one gram-mole for molecular substances (the mass in grams numerically equal to the molecular weight). Volume is not the defining criterion because gas volume varies with state; mass in kilograms is not the convention for defining a mole.

Step-by-Step Solution:

Verification / Alternative check:
The SI Brochure defines a mole by fixing NA to an exact value; the concept remains identical across phases.

Why Other Options Are Wrong:

• Gram or kilogram: mass units, not amount-of-substance definitions.
• Litre/cubic metre: volumes depend on T and P; not a universal mole definition.

Common Pitfalls:
Equating the 'molar volume' at STP with the definition of a mole; confusing mass with amount of substance.

Final Answer:
Gram-mole (1 mol)