First Class Chemistry
1
THE INTERNATIONAL SYSTEM OF UNITS
Lec. 1
By: Dr. Tamathir Abbas 29/11/2015
THE INTERNATIONAL SYSTEM OF UNITS (SI)
The first international system of measurement was the metric
system. It was adopted by the United States and 16 other nations in 1875.
This is a decimal system, which means that larger and smaller parts of the
basic units are obtained by multiplying or dividing by ten. The metric
system was extensively revised in 1960 and was given a new name,
"International System of Units." Its official abbreviation is
SI
.
Each has a characteristic name obtained by adding a prefix to the basic
name of the unit. The prefix
tells us how many times the basic
unit has been divided or multiplied by ten. The names of the
most commonly used prefixes are given in Table 1-1.
Length
The basic unit of length or distance in SI is the meter.
The most
commonly used fractions of the meter are the centimeter and the
millimeter.
Other fractions and multiples of the meter are given in Table
1-2.
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Mass
☻Mass is defined as the amount of material in a certain object compared
to a reference standard mass.
☻Weight is the measurement of the attraction of the earth's gravity for
the mass of the object.
☼ The mass of an object is the same on the moon as on the earth, but it
weight less because of the lower gravity of the moon.
☼ The SI unit of mass is the kilogram (kg). The most commonly used
fractions are the gram (g), the milligram (mg), and the microgram ( µg).
H.W
EXERCISE 1-1 Construct a table similar to Table 1-2 indicating the relationship
Tg, Gg, Mg, kg, g, dg, eg, mg, jug, ng, and pg.
Volume
The SI unit of volume is the cubic meter. Its symbol is m
3
.
☺ A cubic meter is a very large unit and consequently it is not used very
often by chemists. The liter is the most commonly used unit of volume.
The liter, whose symbol is L, is defined in terms of SI units as 0.001 m
3
,
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or 1000 L = 1 m
3
Commonly used fractions of the liter are the milliliter
(mL).
The various SI units of volume are summarized in Table 1-3.
H.W
EXERCISE 1-2 Construct a Table similar to Table 1-2 indicating the relationship
among TL, GL, ML, kL, L, dL, cL, mL, µL, nL, and pL.
Despite attempts to standardize the units of measurement in the
world, systems of measurement other than SI are commonly used. In the
United States, for example, a system that originated in England several
centuries ago is widely used. People who are accustomed to the use of
feet, pounds, and gallons as measures of length, mass, and volume,
respectively, often find it difficult to use SI units. One way to solve this
problem is to convert from one system of units to another. In the next
section we will learn how to do this.
CONVERTING SYSTEMS OF UNITS
To convert from the system of units used in the United States to SI
units, we must know the equivalents of the two systems. Table 1-4 lists
the most widely used equivalents.
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To make use of the information in Table 1-4, we must convert the
equivalents into conversion factors. A conversion factor is simply the
ratio of one unit divided by another. The relationship between any two
units can be expressed as two conversion factors. For example, let us
write the two conversion factors expressing the relationship between
inches and centimeters. According to Table 1-4, 1 in. = 2.54 cm. We
simply divided both sides of the equation by 1 in. to obtain one
conversion factor.
☺This conversion factor tells us that there are 2.54 cm per 1 in. The
other conversion factor can be obtained by dividing both sides of the
equation by 2.54 cm:
☼This conversion factor tells us that there is 1 in. per 2.54 cm. Each
conversion factor contains the two different units. This is an important
part of the ratio, as we will soon learn.
H.W
EXERCISE 1-3 Write two conversion factors expressing the relationship between
each of the following units:
(a) pound and g (b) gallon and L (c) km and miles (d) ounce and g (e) pint and L (f )
yards and m (g) grain and g
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Example 1-1 Convert 5.00 in. to cm.
Solution:
The first step is to write the quantity that you want to convert, including
its units:
5.00 in.
Next, multiply this quantity by the conversion factor that allows you to
cancel all unwanted units:
The inches on top cancel the inches on the bottom, leaving cm as the
desired unit.
♠ What happens if we multiply by the other conversion factor?
The unit in.
2
/cm makes no sense, so we know that we have made a
mistake. Therefore, the first conversion factor is the correct one to use. If
you use this method all the time, you will not make a mistake.
H.W
EXERCISE 1-4
Convert each of the following quantities into its equivalent in
the specified units:
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(a) 1.00m
3
to gal (b) 75.0cm to in. (c) 25.0 oz to g (d) 325yd to m (e) 5.30qt to L (f)
25.0 mi to km (g) 5.00 kg to Ib (h) 16.0pt to L
TEMPERATURE
In the United States, temperatures are measured in degrees
Fahrenheit (°F). In science, however, the international temperature scales
are the Celsius scale and the Kelvin scale.
The Celsius scale (abbreviated °C) uses the freezing point of water
to define 0° and the boiling point of water as 100°. The scale between
these two points is divided into 100 equal degrees.
The SI unit of
temperature is the Kelvin (K). A degree is identical on the Kelvin and
Celsius scales.
H.W
EXERCISE 1-5 Convert each of the following to Kelvin:
(a) 100° C (b) -20° C '(c) 36.9° C (d) 350° C
EXERCISE 1-6 Convert each of the following to degrees Celsius: (a) 298 K (b)
10 K (c) 150 K (d) 1500 K
To convert degrees Fahrenheit to degrees Celsius and vice versa.
Example 1-3 Convert 40° C to °F.