How many atoms of gold are in a 5 gram sample of pure gold?

The answer is 1.505 * 10²² atoms

Avogadro's number is the number of units (atoms, molecules) in 1 mole of substance: 6.02 * 10²³ units per 1 mole

Number of moles (n) is the quotient of mass (m) and molar mass (Mr)

n = m/Mr

m = 5 g

Mr (Au) = 197 g/mol

n = 5g / 197g/mol = 0.025 mol = 2.5 * 0.01 = 2.5 * 10⁻² mol

If 6.02 * 10²³ atoms are in 1 mol, x atoms are in 2.5 * 10⁻² mol:

6.02 * 10²³ atoms : 1 mol = x : 2.5 * 10⁻² mol

x = 6.02 * 10²³ atoms * 2.5 * 10⁻² mol : 1 mol

x = 15.05 * 10²³⁻² atoms = 15.05 * 10²¹ atoms = 1.505 * 10²² atoms