Which of the following would weigh the least?
(Atomic masses C=12, O=16, Na=23)

1. 2 gram atoms of oxygen
2. one mole of sodium
3. 22.4 litres of carbon dioxide at STP
4. 6.023 x 10²² atoms of carbon

6.023 x 10²² atoms of carbon

Reason — To determine which of the options weighs the least, let's calculate the mass of each:

1. 2 gram atoms of oxygen:

• A gram atom of oxygen refers to 1 mole of oxygen atoms, which has a mass of its atomic mass in grams.
• Oxygen has an atomic mass of 16 g/mol.
• Therefore, 2 gram atoms of oxygen would weigh 2 x 16 g = 32 grams.

1. One mole of sodium:

• The atomic mass of sodium is 23 g/mol.
• Thus, one mole of sodium weighs 23 grams.

1. 22.4 litres of carbon dioxide at STP:

• At Standard Temperature and Pressure (STP), 1 mole of any gas occupies 22.4 liters.
• The molecular formula for carbon dioxide is CO₂.
• The molar mass of CO₂ is 12 (for C) + 2 x 16 (for O) = 44 grams/mol.
• Therefore, 22.4 litres of carbon dioxide weighs 44 grams.

1. 6.023 x 10²² atoms of carbon:

• Avogadro's number is 6.023 x 10²³, which is the number of atoms in 1 mole.
• Therefore, 6.023 x 10²² atoms of carbon is $\dfrac{1}{10}$ of a mole of carbon.
• The atomic mass of carbon is 12 g/mol.
• Therefore, $\dfrac{1}{10}$ mole of carbon has a mass of $\dfrac{12}{10}$ = 1.2 grams.

Comparing the masses:

1. 2 gram atoms of oxygen = 32 grams
2. One mole of sodium = 23 grams
3. 22.4 litres of carbon dioxide at STP = 44 grams
4. 6.023 x 10²² atoms of carbon = 1.2 grams

Hence, out of the given options, 6.023 x 10²² atoms of carbon will weigh the least.