Calculate the mass fractions of the elements that make up Cu (NO3) 2.

Formula for calculating the mass fraction of an element in a substance:
w = n * Ar (chemical element) / Mr (substance), where n is the number of atoms of a given chemical element.

1) Find the relative molecular weight of the substance (Cu (NO3) 2):
Mr (Cu (NO3) 2) = Ar (Cu) + 2 * Ar (N) + 6 * Ar (O) = 64 + 28 + 96 = 188
2) Find the mass fraction of this element in the substance:
w (Cu) = 64/188 = 0.34 or 34%

w (N) = 28/188 = 0.15 or 15%

w (O) = 96/188 = 0.51 or 51%

Answer: Mass fraction of copper in Cu (NO3) 2 34%.

Mass fraction of nitrogen in Cu (NO3) 2 15%.

Mass fraction of oxygen in Cu (NO3) 2 51%.