We already know that number of moles is 0.0025 mol.
So if we know the volume of the gas at STP and the number of moles we can get the mass of the gas.
V = 0.056 L
1 mol = 22.4 L
Molar mass of He = 4.0026 g/mol
So to calculate the weight we need number of moles and molar mass.
weight = 0.0025 mol x 4.0026 g/mol
= 0.01 g
