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Common Units

Introduction

The following units are common in chemistry problems.

 UnitAbbreviation
Length:meterm
Mass:gramg
Volume:literL
Density:g/mL or g/cm3--
Concentration:molarityM
Partial Pressure:atmosphereatm
Amount:molemol


Prefixes

The following prefixes are useful abbreviations when working with very small or very large numbers. You might find it useful to memorize the most common prefixes: M, k, m, and .

prefixnamefactor
Ttera1012
Ggiga109
Mmega106
kkilo103
        
prefixnamefactor
mmilli10-3
micro10-6
nnano10-9
ppico10-12
ffemto10-15

Examples: One nm (nanometer) is 10-9 m (that's small), and 2 kg is 2000 g (that's about a six-pack of your favorite soda).


The Mole

The "mole" is a fixed number of something. That number is 6.022142x1023. This number is called the Avogadro constant or Avogadro's number.

If you have one mole of fish, you have 6.022x1023 fish (that's a lot of fish). The mole is not the most useful unit for describing the number of fish in a supermarket.

So why do we work in units of moles? Because atoms and molecules are very small with very small masses. You would not notice if you had 1 or 10 or 100 gold atoms in your hand. You would notice, and you might be very happy, if you had one mole of gold atoms in your hand. Look at a periodic table and decide just how many grams of gold you would have?


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