How many grams of water do you have to decompose if you want to prepare 34.7 g of oxygen?

Try this solution:

The equation of the reaction is:

2H₂O⇒2H₂↑+O₂↑ where m (O₂) = 34.7gr.

M (O₂) = 16*2=32 (gr./mol);

ν (O₂) = m (O₂) / M (O₂) = 34.7/32=1.084375 (mol).

According to the equation of the reaction ν (H₂O) = 2ν (O₂) = 2*1.084375=2.16875 (mol);

M (H₂O) = 2+16=18 (gr./mol).

m (H₂O) = M (H₂O) * ν (H₂O) = 18*2.16875=39.0375 gr.

Answer: 39.0375 gr.