Question

The Avogadro constant $6.022140857 \times 10^{23$ is defined as the number of atoms present in exactly 12 g of ..........}

• A. Carbon-12
• B. Oxygen-16
• C. Nitrogen-14
• D. Carbon-14

Hint:

The definition of a mole and Avogadro's constant is inextricably linked to Carbon-12 as the reference isotope.

Answer 1

The Correct Option is A

Step 1: Understand the definition of Avogadro's constant.
Avogadro's constant ($N_A$) is a fundamental physical constant that represents the number of constituent particles (usually atoms or molecules) in one mole of a substance. Step 2: Recall the historical and standard definition.
Historically and by current international agreement, a mole is defined as the amount of substance of a system that contains $6.022140857 \times 10^{23}$ elementary entities (atoms, molecules, ions, etc.). This number is specifically related to the number of atoms in exactly 12 grams of carbon-12 ($^{12}\text{C}$). Carbon-12 was chosen as the reference standard for atomic mass and the definition of the mole. Step 3: Evaluate the given options based on this definition.

(1) Carbon-12: This is the internationally accepted reference isotope used in the definition of the mole and Avogadro's constant.
(2) Oxygen-16: While Oxygen-16 is a common isotope, it is not the standard used for defining Avogadro's constant in terms of 12 grams.
(3) Nitrogen-14: Nitrogen-14 is not used in the definition of Avogadro's constant in this context.
(4) Carbon-14: Carbon-14 is an isotope of carbon, but Carbon-12 is the specific isotope used for the definition. Step 4: Conclude the correct element.
The Avogadro constant is defined based on the number of atoms present in exactly 12 g of Carbon-12. (1) Carbon-12