Question

How many total atoms are in 0.240 g of P2O5?

Answer 1

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