Measurements and Units
In April 2003, the US Pharmacopeia, a national organization that establishes quality standards for medications, reported a case in which a physician ordered “morphine [a powerful painkiller] 2–3 mg IV [intravenously] every 2–3 hours for pain.” A nurse misread the dose as “23 mg” and thus administered approximately 10 times the proper amount to an 8-year-old boy with a broken leg. The boy stopped breathing but was successfully resuscitated and left the hospital three days later.
Quantities and measurements are not only important in medicine, but also in our everyday lives. The posted speed limits on roads and highways, such as 55 miles per hour (mph), are quantities we might encounter all the time. Both parts of a quantity, the amount (55) and the unit (mph), must be properly communicated to prevent potential problems. A coffee maker’s instructions tell you to fill the coffeepot with 4 cups of water and use 3 scoops of coffee. When you follow these instructions, you are making a note of the units (cups or scoops) and measuring the appropriate quantity. In chemistry and in cooking, the abilities to communicate, interpret, and measure quantities are necessary skills. You will practice these skills in this class so that errors—from homework mistakes to traffic tickets to more serious consequences—can be avoided.
In chemistry and in cooking, we measure the properties of matter and express these measurements as quantities. A quantity is an amount of something and consists of a number and a unit. The number tells us how many (or how much), and the unit tells us what the scale of measurement is. For example, when a volume is reported as “4 cups,” we know that the quantity has been expressed in units of cups and that the number of cups is 4. If your neighbor asks to borrow some sugar, and you ask how much sugar, and they say “4” without specifying a unit, you do not know whether your neighbor requires 4 teaspoons, 4 tablespoons, 4 cups, 4 liters, 4 bushels, etc. Both a number and a unit must be included to express a quantity properly.
To use chemistry in the kitchen and in everyday life, we need a clear understanding of the units we work with in chemistry. This chapter examines the different types of units utilized in chemistry and how to use them.
Example: Measurements and Units
Identify the number and the unit in each quantity.
- one dozen eggs
- 2.54 centimeters
- a box of pencils
- 88 meters per second
Solution
- The number is one, and the unit is dozen eggs.
- The number is 2.54, and the unit is centimeter.
- The number 1 is implied because the quantity is only a box. The unit is box of pencils.
- The number is 88, and the unit is meters per second. Note that in this case the unit is actually a combination of two units: meters and seconds.
Attributions
This page is based on “Chemistry 2e” by Paul Flowers, Klaus Theopold, Richard Langley, William R. Robinson, PhD, Openstax which is licensed under CC BY 4.0. Access for free at https://openstax.org/books/chemistry-2e/pages/1-introduction
This page is based on “Chemistry of Cooking” by Sorangel Rodriguez-Velazquez which is licensed under CC BY-NC-SA 4.0. Access for free at http://chemofcooking.openbooks.wpengine.com/
This page is based on “The Basics of General, Organic, and Biological Chemistry” by David W Ball, John W Hill, Rhonda J Scott, Saylor which is licensed under CC BY-NC-SA 4.0. Access for free at http://saylordotorg.github.io/text_the-basics-of-general-organic-and-biological-chemistry/index.html
