The mass of oxygen required to prepare 2 moles of water is

1 mole ${\mathrm{H}}_2\mathrm{O}$ contain 16 gram of oxygen.

Mass of oxygen required to prepare 2 moles of water is

$\frac{61\mathrm{g} \mathrm{of} \text{'}\mathrm{O}\text{'}}{1\mathrm{mol} \mathrm{of} {\mathrm{H}}_2\mathrm{O}}\times 2mol of H_{2}O=32g of \text{'}O\text{'}$