3.3: Units for Measurements
- Page ID
- 283821
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Identify the appropriate metric unit for measuring items of various dimensions.
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Use metric and English units in unit analysis problems.
Measurements provide the macroscopic information that is the basis of most of the hypotheses, theories, and laws that describe the behavior of matter and energy in both the macroscopic and microscopic domains of chemistry. Every measurement provides three kinds of information: the size or magnitude of the measurement (a number); a standard of comparison for the measurement (a unit); and an indication of the uncertainty of the measurement. While the number and unit are explicitly represented when a quantity is written, the uncertainty is an aspect of the measurement result that is more implicitly represented and will be discussed later.
The number in the measurement can be represented in different ways, including decimal form and scientific notation. For example, the maximum takeoff weight of a Boeing 777-200ER airliner is 298,000 kilograms, which can also be written as 2.98 \(\times\) 105 kg. The mass of the average mosquito is about 0.0000025 kilograms, which can be written as 2.5 \(\times\) 10−6 kg.
Units, such as liters, pounds, and centimeters, are standards of comparison for measurements. When we buy a 2 liter bottle of a soft drink, we expect that the volume of the drink was measured, so it is two times larger than the volume that everyone agrees to be 1 liter. The meat used to prepare a 0.25 pound hamburger is measured so it weighs one fourth as much as 1 pound. Without units, a number can be meaningless, confusing, or possibly life threatening. Suppose a doctor prescribes phenobarbital to control a patient’s seizures and states a dosage of “100” without specifying units. Not only will this be confusing to the medical professional giving the dose, but the consequences can be dire: 100 mg given three times per day can be effective as an anticonvulsant, but a single dose of 100 g is more than 10 times the lethal amount.
We usually report the results of scientific measurements in SI units, an updated version of the metric system, using the units listed in Table \(\PageIndex{1}\). Other units can be derived from these base units. The standards for these units are fixed by international agreement, and they are called the International System of Units or SI Units (from the French, Le Système International d’Unités). SI units have been used by the United States National Institute of Standards and Technology (NIST) since 1964.
| Property Measured | Name of Unit | Symbol of Unit |
|---|---|---|
| length | meter | m |
| mass | kilogram | kg |
| time | second | s |
| temperature | kelvin | K |
| amount of substance | mole | mol |
Metric System
Sometimes we use units that are fractions or multiples of a base unit. In the metric system these fractions or multiples are always powers of 10. Fractional or multiple SI units are named using a prefix and the name of the base unit. For example, a length of 1000 meters is also called a kilometer because the prefix kilo means “one thousand,” which in scientific notation is 103 (1 kilometer = 1000 m = 103 m).
Figure \(\PageIndex{1}\): Prefixes used with SI and metric units. Each step on the number line represents a factor of 10.
The number line in Figure \(\PageIndex{1}\) can be used to determine the relationship between units with the same base unit. Meters is a base unit; it is abbreviated m with no prefix. Kilometers is abbreviated km. Meters and kilometers are three steps apart on the number line. This corresponds to three factors of ten or 10 x 10 x 10 = 1000. The smaller units are written to the left on the number line in Figure \(\PageIndex{1}\). It takes more of the smaller unit to equal a larger unit. Thus, it takes 1000 m to equal the length of 1 km.
What is the relationship between microliters (\(\mu\)L and milliliters (mL)?
Solution
- Check that the two units have the same base unit. The number line cannot be used for units with different base units. In this case both units end with L, the base unit liters, so we can proceed.
- Find the prefixes on the number line and count how many steps it take to get from one to the other. It takes three steps to get from micro to milli.
- Each step is worth a factor of ten so three steps is 10 x 10 x 10 = 1000. In scientific notation this can be written as 1 x 103 where the exponent on the ten is the number of steps.
- There are many of the smaller unit in one of the larger unit, so the relationship is 1000 \(\mu\)L = 1 mL.
What is the relationship between centimeters and kilometers?
- Answer
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1 x 105 cm = 1 km (or 100,000 cm = 1 km)
What is the relationship between centiliters and deciliters?
- Answer
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10 cL = 1 dL
SI Base Units
The initial units of the metric system, which eventually evolved into the SI system, were established in France during the French Revolution. The original standards for the meter and the kilogram were adopted there in 1799 and eventually by other countries. This section introduces four of the SI base units commonly used in chemistry. Other SI units will be introduced in subsequent chapters.
Length
The standard unit of length in both the SI and original metric systems is the meter (m). A meter was originally specified as 1/10,000,000 of the distance from the North Pole to the equator. It is now defined as the distance light in a vacuum travels in 1/299,792,458 of a second. A meter is about 3 inches longer than a yard (Figure \(\PageIndex{2}\)); one meter is about 39.37 inches or 1.094 yards. Longer distances are often reported in kilometers (1 km = 1000 m = 1 x 103 m), whereas shorter distances can be reported in centimeters (1 cm = 0.01 m = 1 x 10−2 m) or millimeters (1 mm = 0.001 m = 1 x 10−3 m).
Mass
The standard unit of mass in the SI system is the kilogram (kg). A kilogram was originally defined as the mass of a liter of water (a cube of water with an edge length of exactly 0.1 meter). In 1889, it was redefined by a certain cylinder of platinum-iridium alloy, which was kept in France (Figure \(\PageIndex{3}\)). Any object with the same mass as this cylinder was said to have a mass of 1 kilogram. (This led to uncertainties unacceptable to the precision of modern instrumentation. Since May 20, 2019, a new definition has be used for the kilogram, based on the unchanging Planck's constant.).
One kilogram is about 2.2 pounds. The gram (g) is exactly equal to 1/1000 of the mass of the kilogram (1 x 10−3 kg).
Figure \(\PageIndex{3}\): This replica prototype kilogram is housed at the National Institute of Standards and Technology (NIST) in Maryland. (credit: National Institutes of Standards and Technology).
Temperature
Temperature is an intensive property. The SI unit of temperature is the kelvin (K). The IUPAC convention is to use kelvin (all lowercase) for the word, K (uppercase) for the unit symbol, and neither the word “degree” nor the degree symbol (°). The degree Celsius (°C) is also allowed in the SI system, with both the word “degree” and the degree symbol used for Celsius measurements. Celsius degrees are the same magnitude as those of kelvin, but the two scales place their zeros in different places. Water freezes at 273.15 K (0 °C) and boils at 373.15 K (100 °C) by definition, and normal human body temperature is approximately 310 K (37 °C). The conversion between these two units and the Fahrenheit scale will be discussed later in this chapter.
Time
The SI base unit of time is the second (s). Small and large time intervals can be expressed with the appropriate prefixes; for example, 3 microseconds = 0.000003 s = 3 \(\times\) 10−6 and 5 megaseconds = 5,000,000 s = 5 \(\times\) 106 s. Alternatively, hours, days, and years can be used.
Volume
Volume is the measure of the amount of space occupied by an object. The standard SI unit of volume is defined by the base unit of length (Figure \(\PageIndex{4}\)). The standard volume is a cubic meter (m3), a cube with an edge length of exactly one meter. To dispense a cubic meter of water, we could build a cubic box with edge lengths of exactly one meter. This box would hold a cubic meter of water or any other substance.
A more commonly used unit of volume is derived from the decimeter (0.1 m, or 10 cm). A cube with edge lengths of exactly one decimeter contains a volume of one cubic decimeter (dm3). A liter (L) is the more common name for the cubic decimeter. One liter is about 1.06 quarts. A cubic centimeter (cm3) is the volume of a cube with an edge length of exactly one centimeter. The abbreviation cc (for cubic centimeter) is often used by health professionals. A cubic centimeter is also called a milliliter (mL) and is 1/1000 of a liter.
<
What type of measurement does each of the following correspond to? Answer options are: length, mass, temperature, time, volume.
- 4.5 g
- 1500 mL
- 225 ms
- Answer
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As you go through the course, associate units with the type of measurement they correspond to. This will help you interpret word problems.
- 4.5 g = mass. The unit g is grams.
- 1500 mL = volume. The unit mL is milliliters.
- 225 ms = time. The unit ms is milliseconds.
Summary
Measurements provide quantitative information that is critical in studying and practicing chemistry. Each measurement has an amount, a unit for comparison, and an uncertainty. Measurements can be represented in either decimal or scientific notation. Scientists primarily use the SI (International System) or metric systems. We use base SI units such as meters, seconds, and kilograms, as well as derived units, such as liters (for volume) and g/cm3 (for density). In many cases, we find it convenient to use unit prefixes that yield fractional and multiple units, such as microseconds (10−6 seconds) and megahertz (106 hertz), respectively.
Glossary
- Celsius (°C)
- unit of temperature; water freezes at 0 °C and boils at 100 °C on this scale
- cubic centimeter (cm3 or cc)
- volume of a cube with an edge length of exactly 1 cm
- cubic meter (m3)
- SI unit of volume
- kelvin (K)
- SI unit of temperature; 273.15 K = 0 ºC
- kilogram (kg)
- standard SI unit of mass; 1 kg = approximately 2.2 pounds
- length
- measure of one dimension of an object
- liter (L)
- (also, cubic decimeter) unit of volume; 1 L = 1,000 cm3
- meter (m)
- standard metric and SI unit of length; 1 m = approximately 1.094 yards
- milliliter (mL)
- 1/1,000 of a liter; equal to 1 cm3
- second (s)
- SI unit of time
- SI units (International System of Units)
- standards fixed by international agreement in the International System of Units (Le Système International d’Unités)
- unit
- standard of comparison for measurements
- volume
- amount of space occupied by an object
Contributors
Paul Flowers (University of North Carolina - Pembroke), Klaus Theopold (University of Delaware) and Richard Langley (Stephen F. Austin State University) with contributing authors. Textbook content produced by OpenStax College is licensed under a Creative Commons Attribution License 4.0 license. Download for free at http://cnx.org/contents/85abf193-2bd...a7ac8df6@9.110).
- Christy VanRooyen, Oregon Tech
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