SI Units/Conversions:
By Roy Kim

Conversions, 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 1018 E 1,000,000,000,000,000,000
peta 1015 P 1,000,000,000,000,000
tera 1012 T 1,000,000,000,000
giga 109 G 1,000,000,000
mega 106 M 1,000,000
kilo 103 k 1,000
hecto 102 h 100
deka 101 da 10
-- 100 -- 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 20.0o Fahrenheit to Kelvin.

  1. First, convert the Fahrenheit into Celsius. This yields -6.67o C.
  2. Convert the Celsius to Kelvin by adding 273.15.
  3. Answer is: 266.63o 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!

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