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The International System of Units

Quantity Name of Unit Unit Symbollength meter (metre) m mass kilogram kg time second s temperature kelvin K (case sensitive) amount of substance mole mol

Like earlier versions of the metric system, the SI units can be designated as decimal fractions or multiples by the use of appropriate prefixes. The acceptable SI prefixes are given in the table below.

Factor Prefix Symbol Factor Prefix Symbol10e-15 femto f 10e12 tera T 10e-12 pico p 10e9 giga G 10e-9 nano n 10e6 mega M 10e-6 micro() 10e3 kilo k 10e-3 milli m 10e-2 centi c

*N.B. Whenever exponents are used with SI prefixes on either
base units or derived units, the exponent applies to the prefix as well
as to the unit. For example, nm ^{2}, or square nanometer, is interpreted
as (nm)^{2} rather than n(m^{2}). Any prefix can be applied
to any base unit except the kilogram; the kilogram takes prefixes
as if the base unit were the gram. As a consequence 10^{-6} kg
is written as 1 milligram (mg) rather than 1 microkilogram.*

The great advantage of the SI over other systems of units is that when any physical quantity whatever is written out in the SI base units or

Example. The mass of a sample of pure rhombic sulfur was 150.637 g and
the volume of water it displaced was 72.8 mL. The density of sulfur is
then (150.637 g)/(72.8 mL) = 2.07 g/mL, or g/cm^{3}, or kg/dm^{3},
or kg/L.

You need to be familiar with the km, m, cm, mm, m or micron, and a nanometer. Also, 10 nm are equal to 1 angstrom (A).

**MASS**

The SI unit of mass is the kilogram, a fundamental
unit of the SI. The kilogram was once
defined as the mass of one cubic decimetre (L) of water. Since 1901 it
is by definition the mass of the international prototype of the kilogram,
a platinum-iridium mass which is stored at Sevres in France.

An interesting fact about the kilogram is that it is the only SI base
unit to incorporate a prefix. The kilogram is the only SI unit based on
a finite amount of material rather than some physical parameter.

You need to be familar with the kg, g, mg, g and ng

The kelvin (which is correctly written without a degree sign) is used
for measuring both temperature and temperature interval; thus one can say,
"The temperature is 300 K" or "This pan is 20 K hotter than that one."
Temperatures in kelvin can only be positive and so they require no sign.
The kelvin scale of temperature is also known as the absolute scale and
the thermodynamic scale. Conversion factors between temperatures in degrees
Celsius (^{o}C) and temperatures in kelvin are:

Temperature (^{o}C) + 273.15 (exactly)
= temperature (K) (memorize this) |

You need to be familiar with a mol, mmol, mol, nmol and a pmol.

1) Liter: symbol = L.

2) cubic centimeter: symbol = cm^{3}. Often used for measuring
the volume of solids, one cm^{3} equals

one milliliter (mL). The mL has emerged as the common measure of volume.

3) Ångström: symbol = Å. One Å equals 10¯^{8}
cm. ** THERE ARE 10Å IN ONE NANOMETER**.