Calculate how much of a substance is contained in a sample of copper weighing 6.4 g.

n is the amount of substance, n = m / M.
m is the mass of the substance, given according to the condition of the problem, 6.4 g;
M is the molar mass of the substance, (the sum of the atomic masses). In this case, Copper is a simple substance, so the molar mass will be equal to the atomic mass, which can be viewed in the periodic system of D.I. Mendeleev.
Copper – Cu, Ar (Cu) = 64.
n = 6.4 / 64 = 0.1 mol.
Answer: the amount of substance that is contained in a sample of copper weighing 6.4 g is equal to 0.1 mol.