3 Cu + 8HNO3 → 3 Cu(NO3)2 + 2 NO + 4 H2O
n(HNO3)=6.3mol
unknown/known=4/8=1/2
n(H2O)=n(HNO3)/2=3.15mol
m(H2O)=nxM=3.15x(1+1+16)=56.7g
Let me know if its wrong
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