Balance the following chemical equation?

Question

Cu + HNO3 --> Cu(NO3)2 + NO2 + H2O

Answer 1

Cu + 4 HNO3 --> Cu(NO3)2 + 2 NO2 + 2 H2O

Answer 2

First identify the reduction/oxidation pairs
Cu oxidized from 0 to Cu+2
N reduced from +5 to +4
Set up half equations, using H2O and H+ where necessary to balance mass, and finding the excess electrons in the half equations
For Cu
Cu(0) = Cu+2 + 2e
2H+ + NO3- + e = NO2 + H2O
We double the second half equation to balance electrons, then add both half equations.
Cu + 4H+ + 2 NO3- = 2NO2 + 2H2O + Cu+2
We add additional nitric acid to meet the Cu+2 needs.
Cu + 4HNO3 -> 2 NO2 + 2 H2O + Cu(NO3)2