Question 1.ex.2: How many grams of aspirin are contained in a 325-mg tablet?

Step [1] Identify the original quantity and the desired quantity, including units.
• In this problem the original quantity is reported in milligrams and the desired quantity is in grams.

325 mg           ? g
original quantity      desired quantity

Step [2] Write out the conversion factor(s) needed to solve the problem.
• We need a conversion factor that relates milligrams and grams (Table 1.3). Since the unwanted unit is in milligrams, choose the conversion factor that contains milligrams in the denominator so that the units cancel.

Table 1.3 Summary of the Common Metric Units of Length, Mass, and Volume

• Sometimes one conversion factor is all that is needed in a problem. At other times (Section 1.7C) more than one conversion factor is needed.
• If the desired answer has a single unit (grams in this case), the conversion factor must contain the desired unit in the numerator and the unwanted unit in the denominator.

Step [3] Set up and solve the problem.
• Multiply the original quantity by the conversion factor to obtain the desired quantity.

Step [4] Write the answer using the correct number of significant figures and check it by estimation.
• Use the number of significant figures in each inexact (measured) number to determine the number of significant figures in the answer. In this case the answer is limited to three significant figures by the original quantity (325 mg).
• Estimate the answer using a variety of methods. In this case we knew our answer had to be less than one, since it is obtained by dividing 325 by a number larger than itself.