Question

Determine the number of molecules in a 261 mg dose of aspirin, molecular formula C9H8O4.

Answer 1

Molar mass of Aspirin = 180 g / mol

Divide Avogadro's number (6.022 * 10^23) by the grams per mole to find the molecules per gram.
                                 = 6.022 * 10^23 / 180 g / mol

                                 = 3.345 * 10^21 molecules per gram.

Now convert to milligrams (1000mg/1g) to get 3.345 * 10^18 molecules/mg of Aspirin.

Multiply by 261mg and the final answer is 8.7319 * 10^20 molecules in 216 mg of Aspirin in that tablet.