A helium balloon contains 0.40 g of helium. given the molecular weight of helium is 4.0 g/mol, how many moles of helium does it contain?

Answer is 0.10 mol.

Explanation;

n = m/M

Where n is number of moles (mol), m is the mass of substance (g) and M is the molar mass of the substance (g/mol).

n = ?

m = 0.40 g

M = 4.0 g/mol

From substitution,

n = 0.40 g / 4.0 g/mol

n = 0.10 mol

Hence, the balloon has 0.10 mol of He.