SI Units/Conversions:
By Roy KimConversions, which you will hopefully find very easy, serve as the foundation
for the rest of chemistry. Because there are so many ways to measure length, weight, etc.,
you must learn how to convert one unit of measurement to another. By the time you reach
the end of chemistry, thes conversions will have become second nature.
SI Units
In order to make measuring and recording easier for the common chemistry student (and
brilliant scientists) a group of scientists decided to implement the SI system
as the official standard of measuring in the world. The system is based on the metric
system, and all units are derived from the metric system. The metric system uses the basic
units of measurement listed in the following table.
Base Units of the SI System 

MEASUREMENT 
NAME OF UNIT 
SYMBOL 
Length 
meter 
m 
Mass 
kilogram 
kg 
Time 
second 
s 
Electric current 
ampere 
A 
Temperature 
kelvin 
K 
Luminous intensity 
candela 
cd 
Amount of substance 
mole 
mol 
All other units of measurement in the netric system are derived by
multiplying the base units by some factor of ten. Prefixes are added to the names of each
unit to indicate the factor of ten by which to multiply. The prefixes, which are the same
for each unit, can be found below.
Prefixes Used In Metric System 

PREFIX 
FACTOR 
SYMBOL 
AMOUNT 
exa 
10^{18}

E 
1,000,000,000,000,000,000 
peta 
10^{15}

P 
1,000,000,000,000,000 
tera 
10^{12}

T 
1,000,000,000,000 
giga 
10^{9}

G 
1,000,000,000 
mega 
10^{6}

M 
1,000,000 
kilo 
10^{3}

k 
1,000 
hecto 
10^{2}

h 
100 
deka 
10^{1}

da 
10 
 
10^{0}

 
1 
deci 
10^{1}

d 
0.1 
centi 
10^{2}

c 
0.01 
milli 
10^{3}

m 
0.001 
micro 
10^{6}

m 
0.000001 
nano 
10^{9}

n 
0.000000001 
pico 
10^{12}

p 
0.000000000001 
femto 
10^{15}

f 
0.000000000000001 
atto 
10^{18}

a 
0.000000000000000001 
So, quick practice:
If there are 1000 grams of paper clips, how many hecto grams of
paper clips are present?
1000 grams is 100 deka grams. 100 deka grams is 10 hectograms. So, you have 10
hectograms.
Conversions:
Now, not all measurements will be in an SI unit. A good example is temperature. It is
crucial to always use SI units in any formula in chemistry because most formulas relate to
measurements taken in SI units, and use of another type of unit in these formulas would
result in an incorrect answer. Often times temperature will be made in fahrenheit or
celsius. If you have the former, there is a long process in obtaining the kelvin
equivalents.
Let's try converting 20.0^{o} Fahrenheit to Kelvin.

First, convert the Fahrenheit into Celsius. This yields 6.67^{o} C.
 Convert the Celsius to Kelvin by adding 273.15.

Answer is: 266.63^{o} K is your final answer!
Often time, people will ask, "How do you know when to divide or multiple?"
Let's look at simple algebra.
Let's say you have a variable "x". You want to achieve an answer of
"2". So, by what would you multply that "x" in order to get
"2?" You would multiply by 2/x * 2/x = 2x/x = 2. See? You would multiple 2/x,
not x/2. So, when you are trying to convert between diferent units, divide your original
unit by the original unit over the new unit.
400 torr * (1 atm/760 torr) ===> the torrs will cancel, out, giving you atm.
Eventually, you will learn more conversions. Go try the practice now!