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ChemWiki: The Dynamic Chemistry E-textbook > Analytical Chemistry > Quantifying Nature > Units of Measure > SI Units

SI Units

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The International System of Units (SI)  is  system of units of measurements  that is widely used all over the world. This modern form of the Metric system is based around the number 10 for convienece.  A set unit of prefixes  have been established and are known as the SI prefixes or the metric prefixes (or units).  The prefixes indicate whether the unit is a multiple or a fraction of the base ten. It allows the reduction of zeros of a very small number or a very larger number such as  0.000000001 meter and 7,500,000 Joules into nanometer and 7.5 Megajoules respectively.  These SI prefixes also have a set of symbols that precede unit symbol.

Introduction

However countries such as the United States, Liberia, and Berma have not officially adopted the International System of Units as their primary system of measurements. Since the SI Units are nearly globally though, the scientific and mathematical field will use these SI units in order to provide ease between  the sharing data with one another because of  a common set of measurements. Be prepared to use these units in future chemistry  (and other) courses.

SI Base Units

The SI contains seven BASE UNITS that each represent a different kind of physical quantity. These are commonly used as a convention. 

PHYSICAL QUANTITY NAME OF UNIT  ABBREVIATION
Mass Kilogram kg
Length Meter m
Time Second s
Temperature Kelvin K
Amount of Substance Mole mol
Electric Current Ampere A
Luminous Intensity Candela cd

SI Derived Units

Derived Units are created by mathematical relationships between other Base Units and are expressed in a combination of fundamental and base quantities. 

DERIVED QUANTITY  NAME ABBREVIATION
Area Square Meter m2

Volume

Cubic Meter m3
Mass Density Kilogram Per Cubic Meter kg/m3
Specific Volume Cubic Meter Per Kilogram m3/kg
Celsius Temperature degree Celsius oC

Unit Prefixes

Metric units use a prefix, used for conversion from or to an SI unit.  Below is a chart illustrating how prefixes are labeled in metric measurements. The chart may be printed for quick reference, if needed.

SYMBOL PREFIX  MULTIPLICATION FACTOR
T Tera 10 12
G Giga 10 9
M Mega 10 6
k Kilo 10 3
h Hecto 10 2
da Deka 10 1
d Deci 10 -1
c Centi 10 -2
m Milli 10 -3
µ Micro 10 -6
n Nano 10 -9
p Pico 10 -12

 

PREFIXES

TERA GIGA MEGA KILO HECTO DEKA DECI CENTI MILLI MICRO NANO PICO
Trillion Billion Million Thousand Hundred Ten Tenth Hundredth Thousandth Millionth Billionth Trillionth

 

Conversion factors are used to convert from one form to another. In many problems, conversions must be used to retrieve the desired SI unit. In these cases the units are specific and the numerical values will vary drastically. 

Temperature

Temperature is usually measured in Celcius, (although the U.S. uses Fahrenheit), but is usually converted to for the Kelvin Scale. 

For Fahrenheit to Celcius:

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For Celcius to Kelvin:

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Reference Points and Ideas:

Melting Point of ice is 0oC = 32oF

Boiling Point of water is 100oC = 212oF

Celcius degrees are recorded and used in labs, but are converted to K for use in calculations in Chemistry. The Kelvin Scale does not use the degree symbol, only K. The Kevlin temperature can only be positive since it is an abosolute value scale

Mass

Mass is usually measured by a sensitive balance machine 

1 kilograms = 2.205 lbs.

(Remember that 1 kg = 1000 grams)

Length

The U.S. usually makes measurements in inches and feets, but the SI system prefers meters as the unit for length.

1 meter = 3.281 feet.

1 inches = 2.54 centimeters

Volume

SI units commonly uses derived units for Volume such as meters cubed to liters. 

1 cm3 (centimeter cubed) = 1mL (mililiter)

1000 cm3 = 1 L = 1 dm3

Energy

1 calorie = 4.184 Joules

Amount of Substance

1 mole = 6.022 x 1023 molecules/atoms

(Avogadro's number)

Practice Problems

Convert to SI Units:

1.     1 Day 4 Hours and 20 Minutes

2.     10.8 Lbs.

3.     58.8 Ft.

4.     10,288 grams

5.     128,968,888 mL

6.     1.4 Degrees Celcius

7.    16.13 Cal

8.    18,888,888 km

Answers given here:

#1-4 http://img708.imageshack.us/img708/2069/001fxh.jpg

#5-8 http://img16.imageshack.us/img16/9512/002gfw.jpg

References

  1. Petrucci, Ralph H. General Chemistry. 9th ed. New Jersey: Pearson Prentice Hall, 2005.
  2. Ryamond, Kenneth W. General, Organic, & Biological Chemistry. 2nd Ed. Danvers, MA: John Wiley & Sons Inc., 2008.

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Last Modified
17:55, 1 Oct 2013

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