Molar Mass of Helium Gas

We use various units to measure the mass or weight of objects. We weigh our apples, oranges, vegetables and grains before buying them. It is easy to comprehend the idea of mass of a ball, apple or planet. There are different instruments for measuring the mass. However, can one place molecules of Helium gas on the grocer’s weighing balance and find how much they weigh?

What is the Molar Mass of Helium Gas

Molecular mass (MM), molecular weight (MW), formula mass (FM) or formula weight (FW) of a substance is the mass of a molecule of that substance. Here, we assume that every molecule that is composed of the same combination of atoms, will have the same mass. The unit of measure used is amu, which stands for atomic mass units.
One atom of carbon-12 (Carbon atoms with 6 neutrons and 6 protons) has a mass of 12 amu. This means 1 amu is about the weight of a neutron. It is evident that it is very small in magnitude. Atomic mass unit can also be expressed  in grams per mole (g/mol). One mole is the number of atoms in 12 grams of carbon-12. This number is called Avogadro's number or Avogadro's constant (N_A) and is equal to 6.022 x 10²³.
Sounds a little confusing? Let us do some math.
1atom of carbon - 12 = 12 amu
1 mole = no. of atoms in 12 g of carbon - 12, which happens to be 6.022 x 10²³ .
1 amu of an element = x g/mol  ( grams per mole - varies with each element).
This is the molar mass of the substance. It is the mass of one mole of the substance in grams, and not mass of one molecule.

Calculate Molar Mass

Helium occurs in natural form as a gas. It is a pure substance, i.e. an element in the periodic table.

One mole of helium atoms = 6.022 x 10²³ Helium atoms.
Number of atoms in m moles = m x  Avagadro’s constant = m x 6.022 x 10²³
So, 10 moles of helium (He) contains 10 x 6.022 x 10²³ = 60.022 x 10²³ helium atoms.
One mole of a pure element = molecular mass of the element.
One mole of molecules = Avogadro’s constant number of molecules.
So one mole of helium molecules =  6.022 x 10²³ molecules of helium.
Molecular mass of helium = Mass of one mole of helium molecules.
The question then is, how many grams of helium fit in one mole? This is the molar mass of Helium gas and happens to be 4.003 g/mol. (grams per mole of helium).