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.
