# Some Important Units of Measurement in Analytical Chemistry

####
**SI
Units**

** Scientists
throughout the world have adopted a standardized system of units known as the International
System of Units (SI).

** This system
(SI) is based on the seven fundamental base units shown in Table (1).

Table (1) |

** Numerous
other useful units, such as volts, hertz, coulombs, and joules, are derived
from these base units.

** To express
small or large measured quantities in terms of a few simple digits, prefixes are
used with these base units and other derived units.

** As shown in Table
(2), these prefixes multiply the unit by various powers of 10.

Table (2) |

** For example,
the wavelength of yellow radiation used for determining sodium by flame
photometry is about 5.9 × 10

^{-7}m, which can be expressed more compactly as 590 nm (nanometers); the volume of a liquid injected onto a chromatographic column is often roughly 50 × 10^{-6}L, or 50 μL (microliters); or the amount of memory on some computer hard disks is about 20 × 10^{9}bytes, or 20 Gbytes (gigabytes)
** In
analytical chemistry, we often determine the amount of chemical species from
mass measurements.

** For mass
measurements, metric units of kilograms (kg), grams (g), milligrams (mg), or
micrograms (μg) are used.

** Volumes of
liquids are measured in units of liters (L), milliliters (mL), microliters
(μL), and sometimes nanoliters (nL). The liter, the SI unit of volume, is
defined as exactly 10

^{-3}m^{3}. The milliliter is defined as 10^{-6}m^{3}, or 1 cm^{3}.**Note:**The ångstrom unit Å is a non-SI unit of length that is widely used to express the wavelength of very short radiation such as X-rays (1 Å = 0.1 nm = 10

^{-10}m). Thus, typical X-radiation lies in the range of 0.1 to 10 Å.

####
**The
Distinction Between Mass and Weight**

** It is
important to understand the difference between mass and weight.

** Mass is an
invariant measure of the quantity of matter in an object.

** Weight is
the force of attraction between an object and its surroundings, principally the
earth. Because gravitational attraction varies with geographical location, the
weight of an object depends on where you weigh it.

** For example,
a crucible weighs less in Denver than in Atlantic City (both cities are at
approximately the same latitude) because the attractive force between the
crucible and the earth is smaller at the higher altitude of Denver. Similarly,
the crucible weighs more in Seattle than in Panama (both cities are at sea level)
because the Earth is somewhat flattened at the poles, and the force of
attraction increases measurably with latitude. The mass of the crucible,
however, remains constant regardless of where you measure it.

** Weight and
mass are related by the familiar expression:

**w = mg**

where w is the
weight of an object, m is its mass, and g is the acceleration due to

gravity.

** A chemical
analysis is always based on mass so that the results will not depend on
locality.

** A balance is
used to compare the mass of an object with the mass of one or more standard masses.
Because g affects both unknown and known equally, the mass of the object is
identical to the standard masses with which it is compared.

** The
distinction between mass and weight is often lost in common usage, and the
process of comparing masses is usually called weighing. In addition, the objects
of known mass as well as the results of weighing are frequently called weights.

** Always bear
in mind, however, that analytical data are based on mass rather than weight.
Therefore, we will use mass rather than weight to describe the quantities of
substances or objects.

** On the other
hand, for lack of a better word, we will use “weigh” for the act of determining
the mass of an object. Also, we will often say “weights” to mean the standard masses
used in weighing.

**Note:**Mass m is an invariant measure of the quantity of matter. Weight w is the force of gravitational attraction between that matter and Earth.

####
**The
Mole**

** The mole
(abbreviated mol) is the SI unit for the amount of a chemical substance. It is
always associated with specific microscopic entities such as atoms, molecules,
ions, electrons, other particles, or specified groups of such particles as
represented by a chemical formula.

** It is the amount
of the specified substance that contains the same number of particles as the
number of carbon atoms in exactly 12 grams of

^{12}C. This important number is Avogadro’s number N_{A}= 6.022 × 10^{23}.
** The molar
mass

*M*of a substance is the mass in grams of 1 mole of that substance.
** We calculate
molar masses by summing the atomic masses of all the atoms appearing in a
chemical formula. For example, the molar mass of formaldehyde CH

_{2}O is:
Thus, 1 mole of
formaldehyde has a mass of 30.0 g, and 1 mole of glucose has a mass of 180.0 g.

**Note:**A mole of a chemical species is 6.022 × 10

^{23}atoms, molecules, ions, electrons, ion pairs, or subatomic particles.

####
**The
Millimole**

** Sometimes it
is more convenient to make calculations with millimoles (mmol) rather than moles.

** The
millimole is 1/1000 of a mole, and the mass in grams of a millimole, the
millimolar mass (m

*M*), is likewise 1/1000 of the molar mass.**1 mmol = 10**

^{-3}mol**10**

^{3}mmol = 1 mol####
**Calculating
the Amount of a Substance in Moles or Millimoles**

** The two
examples that follow illustrate how the number of moles or millimoles of a
species can be determined from its mass in grams or from the mass of a
chemically related species.

**Example (1):**

**Find the number of moles and millimoles of benzoic acid (M = 122.1 g/mol) that are contained in 2.00 g of the pure acid.**

**Solution:**

If we use HBz
to represent benzoic acid, we can write that 1 mole of HBz has a mass of 122.1
g. Therefore,

To obtain the
number of millimoles, we divide by the millimolar mass (0.1221 g/mmol), that
is,

**Example (2):**

**What is the mass in grams of Na**

^{+}(22.99 g/mol) in 25.0 g of Na_{2}SO_{4}(142.0 g/mol)?**Solution:**

The chemical
formula tells us that 1 mole of Na

_{2}SO_{4}contains 2 moles of Na^{+}, that is,
To find the
number of moles of Na

_{2}SO_{4}
Combining this
equation with the first leads to:

To obtain the
mass of sodium in 25.0 g of Na

_{2}SO_{4}, we multiply the number of moles of Na^{+}by the molar mass of Na^{+}, or 22.99 g. And so,
Substituting
the previous equation gives the mass in grams of Na

^{+}:

**Reference:**

*Fundamentals of analytical chemistry / Douglas A. Skoog, Donald M. West, F. James Holler, Stanley R. Crouch. (ninth edition) , 2014 . USA*

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