How many total atoms are in 0.240 g of P2O5?

The answer is 1.02 * 10²⁵ atoms.

1. Calculate molar mass (Mr) of P2O5 which is the sum of atomic masses (Ar) of its elements:

Ar (P) = 31 g/mol

Ar (O) = 16

Mr (P2O5) = 2 * Ar (P) + 5 * Ar (O) = 2 * 31 + 5 * 16 = 62 + 80 = 142 g/mol

2. Calculate number of moles (n) which is the quotient of a sample mass (m) and the molar mass (Mr):

n = m/Mr

m = 0.240 g

Mr = 142 g/mol

n = 0.240 / 142 = 0.0017 mol

3. Avogadro's number is the number of units atoms in 1 mole of substance:

6.023 * 10²³ atoms per 1 mol

x atoms are per 0.0017 mol

6.023 * 10²³ atoms : 1 mol = x atoms : 0.0017 mol

x = 6.023 * 10²³ atoms * 0.0017 mol : 1 mol

x = 0.0102 * 10²³ = 1.02 * 10² * 10²³ = 1.02 * 10²⁵ atoms