Question 5.SP.14: Converting mass to number of molecules: How many molecules a...
Converting mass to number of molecules: How many molecules are contained in a 325-mg tablet of aspirin (C_{9}H_{8}O_{4} , molar mass 180.2 g/mol)?
[1] Identify the original quantity and the desired quantity.
\begin{matrix} \text{325 mg of aspirin} &\text{ ? molecules of aspirin} \\ \text{original quantity} & \text{desired quantity}\end{matrix}
[2] Write out the conversion factors.
• We have no conversion factor that directly relates milligrams to number of molecules. We do know, however, how to relate milligrams to grams, and grams to number of molecules. In other words, we need two conversion factors to solve this problem.
• Choose the conversion factors that place the unwanted units, grams and milligrams, in the denominator so that the units cancel.
[3] Set up and solve the problem.
• Arrange each term so that the units in the numerator of one term cancel the units in the denominator of the adjacent term. The single desired unit, number of molecules, must be located in the numerator of one term.
