How many atoms of gold (au) are present in a sample of 2.5 mol au? a. 1.5 × 1024 atoms au b. 1.5 × 1023 atoms au c. 4.2 × 1024 atoms au d. 4.2 × 1023 atoms au

The answer is A. 1.5 × 1024 atoms Au.

To calculate this, we will use Avogadro's number which is the number of atoms in 1 mole of substance:
6.02 × 10²³ units per 1 mole

So, we need a proportion:
If 6.02 × 10²³ atoms are in 1 mole, how many atoms will be present in 2.5 moles:
6.02 × 10²³ atoms : 1 mole = x : 2.5 mol

After crossing the products:
1 mole * x =  2.5 moles * 6.02 × 10²³ atoms
x = 2.5 * 6.02 × 10²³ atoms
x = 15.05 × 10²³ atoms = 1.505× 10²⁴ atoms = 1.5× 10²⁴ atoms

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