Writing Conversion Factors General, Organic, and Biological Chemistry Fourth Edition Karen Timberlake Chapter 1 Chemistry and Measurements 1.8 Writing Conversion Factors Lectures © 2013 Pearson Education, Inc.

Equalities Equalities use two different units to describe the same measured amount. are written for relationships between units of the metric system; between U.S. units or between metric and U.S. units. Examples:

Equalities and Conversion Factors Equalities are written as a fraction. used as conversion factors. can be represented with one equality in the numerator and the second equality in the denominator. Examples:

Common Equalities

Exact and Measured Numbers in Equalities Equalities between units of the same system are definitions with numbers that are exact. different systems (metric and U.S.) are measurements with numbers that have significant figures. The equality of 2.54 cm = 1 in. is an exception and considered to be exact.

Metric Conversion Factors We can write equalities as conversion factors.

Metric-US Conversion Factors Metric–US conversion factors are written as a ratio with a numerator and denominator. Example:

Equalities on Food Labels The contents of packaged foods in the U.S. are listed in both metric and U.S. units. indicate the same amount of a substance in two different units.

Learning Check Write equalities and conversion factors for each pair of units. 1. liters and mL 2. hours and minutes 3. meters and kilometers

Solution Write equalities and conversion factors for each pair of units.

Conversion Factors in a Problem A conversion factor may be obtained from information in a word problem. is written for that problem only. Example 1: The motorcycle was traveling at a speed of 85 km/h. Example 2: One tablet contains 500 mg vitamin C.

Percent as a Conversion Factor A percent factor uses a ratio of the parts to the whole in a fraction. uses the same units for the parts and whole. uses the value 100 for the whole. can be written as two factors. Example: A food contains 18% (by mass) fat.

Parts per Million, Parts per Billion Parts per Million, ppm is the same as mg per kilogram mg/kg of substance. Parts per Billion, ppb is the same as g per kilogram g/kg of substance. Example: The maximum dose of lead in pottery glaze is 2 ppm.

Chemistry Link to Health Toxicology and Risk-Benefit Assessment One measure of toxicity is the LD50 or lethal dose, which is the concentration of the substance that causes death in 50% of the test animals. A dose is typically measured in ppm (mg/kg) of body mass or ppb (μg/kg).

Chemistry Link to Health Toxicology and Risk-Benefit Assessment

Learning Check Write the equality and conversion factors for each of the following. 1. meters and centimeters 2. jewelry that contains 18% gold 3. one liter of gas is $ 0.95

Solution Write the equality and conversion factors for each of the following.