Why does half a mole of water molecules have more mass than 2 moles of helium atoms?

Mass H2O = 9.01 grams

Mass He = 8.0 grams

The mass per mol of water is more than the mass per mol of helium.

Explanation:

Step 1: Data given

Molar mass water = 18.02 g/mol

Molar mass of helium = 4.0 g/mol

Number of moles H2O = 0.5 moles

Number of moles He = 2.0 moles

Step 2: Calculate mass

Mass = moles * molar mass

Mass H2O = 0.5 moles * 18.02 g/mol

Mass H2O = 9.01 grams

Mass He = 2.0 moles * 4.0 g/mol

Mass He = 8.0 grams

The mass per mol of water is more than the mass per mol of helium.

everything (matter) is made up of molecules (atoms)