A balanced chemical equation is one in which the number of atoms of each element are the same on both the reactant and the product side of the equation. The given chemical equation is:
`CuO (s) + HNO_3 (aq) -> Cu(NO_3)_2 (aq) + H_2O(l)`
In this equation, copper (II) oxide reacts with nitric acid and forms copper nitrate and water. If we check the number of atoms of each element on each side of the equation, we find that the equation is not balanced. This is because there is only 1 nitrogen atom on reactant side, while there are 2 nitrogen atoms on the product side. Similarly, the equation is not balanced in terms of hydrogen and oxygen.
The balanced equation is:
`CuO (s) + 2 HNO_3 (aq) -> Cu(NO_3)_2 (aq) + H_2O (l)`
That is, 1 mole of copper oxide reacts with 2 moles of nitric acid to produce 1 mole of copper nitrate and 1 mole of water. One can check that the number of atoms of each element are the same on both sides of the equation.
Hope this helps.