Question

In which among the given samples, does 6.022 x 10^23 molecules contain ?

• A. 80 g NaOH
• B. 180 g H₂O
• C. 32 g oxygen
• D. 56 g nitrogen

Answer 1

Answer:

C. 32 g oxygen

Understanding Moles and Avogadro's Number

• The core of this question lies in understanding two fundamental concepts in chemistry: Avogadro's Number and the Mole (mol).

Avogadro's Number (N_A)

• Avogadro's Number is approximately 6.022 x 10²³.
• It represents the number of constituent particles (atoms, molecules, ions, electrons, etc.) in one mole of a substance.
• This constant is crucial for relating macroscopic quantities (mass) to microscopic quantities (number of particles).
• Historical Context: The value was first derived by Johann Josef Loschmidt in 1865, but was named after Amedeo Avogadro for his hypothesis regarding the relationship between the volumes of gases and the number of molecules they contain.

The Mole (mol)

• A mole is the SI unit for the amount of substance.
• One mole of any substance contains Avogadro's number (6.022 x 10²³) of particles (atoms, molecules, formula units, etc.).
• The mass of one mole of a substance is equal to its molar mass, expressed in grams per mole (g/mol).
• The molar mass numerically equals the atomic mass (for elements) or molecular/formula mass (for compounds) but with units of grams.

Applying to Oxygen

• The question asks which sample contains 6.022 x 10²³ molecules. This directly corresponds to one mole of molecules.
• Oxygen, as a gas, exists as diatomic molecules, meaning each molecule consists of two oxygen atoms (O₂).
• The atomic mass of a single oxygen atom (O) is approximately 16 atomic mass units (amu).
• To find the molecular mass of an oxygen molecule (O₂), we multiply the atomic mass by 2: 2 x 16 amu = 32 amu.
• Therefore, the molar mass of O₂ is 32 grams per mole (32 g/mol).
• This means that 32 grams of oxygen (O₂) contains one mole of O₂ molecules, which is 6.022 x 10²³ molecules.

Key Points for Competitive Exams

• Remember the value of Avogadro's Number: 6.022 x 10²³.
• Understand that one mole of any substance contains this many particles.
• Know that the molar mass in grams is numerically equal to the atomic/molecular mass in amu.
• Always consider the correct molecular formula (e.g., O is oxygen atom, O₂ is oxygen molecule) when calculating molar mass for gaseous elements like oxygen, hydrogen (H₂), nitrogen (N₂), chlorine (Cl₂), etc.
• This concept forms the basis for stoichiometry calculations in chemistry.