Here's how to calculate the mass of 2.50 mol of Helium:

1. Molar Mass of Helium

* The molar mass of Helium (He) is 4.003 g/mol. This means that one mole of Helium atoms has a mass of 4.003 grams.

2. Calculation

* To find the mass of 2.50 moles of Helium, multiply the molar mass by the number of moles:

Mass = (Molar Mass) x (Number of Moles)

Mass = (4.003 g/mol) x (2.50 mol)

Mass = 10.01 g

Therefore, the mass of 2.50 mol of Helium is 10.01 grams.