Find the total number of atoms in a sample of cocaine hydrochloride, c17h22clno4, of mass 20.5 mg.

First we find the number of moles of cocaine hydrochloride;

Number of moles = mass / molar mass

Mass = 20.5mg = 0.025 g

Molar mass of cocaine hydrochloride = 339.816 g/mol

Number of moles = 0.025 / 339.816

= 7.35692 x 10⁻⁵ moles

In 1 mole there is Avogadro's number of molecules, so in 7.35692 x 10⁻⁵ moles there are;

7.35692 x 10⁻⁵ x 6.02 x 10²³ = 4.429 x 10¹⁹ molecules

And in C₁₇H₂₂ClNO₄, number of atoms = 17C + 22H + 1Cl + 1N + 4O = 45

So, number of atoms in a sample of cocaine hydrochloride = 45 x 4.429 x 10¹⁹ = 1.99 x 10²⁵ atoms