The mass of oxygen formed during decomposition is 180 g of water.

Here is the equation for the decomposition (electrolysis) of water:
2H2O = 2H2 ↑ + O2 ↑

Let’s find the amount of the original water. To do this, we divide the mass of water by the molar mass of water = 180/18 = 10 mol.

Let’s compose and solve the proportion using the above equation.

2 moles of water = 1 moles of oxygen

10 moles of water = x moles of oxygen

X = 5 moles of oxygen

We find the mass of 5 mol of oxygen = 5 * 32 = 160 g.

Answer: 160 g.