What is the number of moles of 'He' in 104 g of helium gas ?

What is the number of moles of ‘He’ in 104 g of helium gas ?

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UPSC CDS-1 – 2022

The number of moles of a substance is calculated by dividing the given mass by its molar mass. The atomic mass of Helium (He) is approximately 4 g/mol. Therefore, 104 g of helium gas contains 104 g / 4 g/mol = 26 moles.

The mole is the SI unit for the amount of substance, defined as containing as many elementary entities as there are atoms in 0.012 kilogram of carbon-12. For elements, the molar mass in grams per mole is numerically equal to the atomic mass in atomic mass units (amu).

Helium is a noble gas and exists as single atoms (monatomic gas), so ‘He’ represents a helium atom, and its molar mass is its atomic mass. If it were a diatomic gas like oxygen (O2), the molar mass would be twice the atomic mass of O.

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