The molar mass of oxygen gas (O2) is 32 g/mol. This means that 64 g of oxygen gas is equal to 64 g / 32 g/mol = 2 mol of oxygen gas.

1 mole of any substance contains 6.022 x 10^23 molecules. Therefore, 2 mol of oxygen gas contains 2 x 6.022 x 10^23 = 1.204 x 10^24 molecules.

So, there are 2 moles and 1.204 x 10^24 molecules in 64 g of oxygen gas.