The mass of one Avogadro number of helium atoms is?

Analyze the question: One Avogadro number of particles (6.02 × 10²³) is exactly one mole. So the question is asking: what is the mass of 1 mole of helium atoms?

Concept: The molar mass of an element in grams per mole is numerically equal to its relative atomic mass.

• Number of moles = mass ÷ molar mass
• 1 mole = 6.02 × 10²³ particles (Avogadro's number)

Solution:

1. Helium has atomic number 2, with 2 protons and 2 neutrons, giving a relative atomic mass of 4 (more precisely 4.00).
2. Molar mass of He = 4.00 g/mol.
3. Helium is a noble gas, so it exists as single atoms (monatomic). That means one mole of He contains 6.02 × 10²³ atoms.
4. Mass = 1 mol × 4.00 g/mol = 4.00 g

Checking the other options:

• A (1.00 g) is the mass of one mole of hydrogen atoms.
• C (8.00 g) would be two moles of helium.
• D (6.00 g) is not the molar mass of helium. It is close to the mass of one mole of carbon-12 halved, which has no relevance here.

Correct answer: B (4.00 g)

Tip: "One Avogadro number = one mole = the atomic mass in grams." For any element, just read the atomic mass off the periodic table and attach "grams." For example, 1 mole of Na is 23 g, 1 mole of C is 12 g, and 1 mole of He is 4 g. Also remember that noble gases are monatomic, so you don't need to double anything as you would with H₂ or O₂.