5) CALCULATIONS USED IN ANALYTICAL CHEMISTRY
Objectives
By the end of this topic the learner should be able to:
1. Explain the role of and apply the Avogadro's number in calculations in the study of chemistry.
2. Describe the methods used to compute the results of a quantitative analysis.
3. State the SI system of units in Chemistry and explain the distinction between mass and weight.
4. Discuss the mole as a measure of the amount of a chemical substance.
5. Express the concentrations of solutions in various ways.
6. Carry out calculations involving chemical stoichiometry.
1) Some Important Units of Measurement
(a) SI Units
Scientists throughout the world have adopted a standardized system of units known as the International
System of Units (SI). This system is based on the seven fundamental base units shown in the Table below.
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. For example:
The wavelength of yellow radiation used for determining sodium by flame photometry is about 5.9 x 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 x 10-6 L, or 50 µL
(microliters);
The amount of memory on some computer hard disks is about 20 x 109 bytes, or 20 Gbytes (gigabytes).
In analytical chemistry, we often determine the amount of chemical species from mass measurements. For
such 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), and sometimes microliters (µL).
The liter, the SI unit of volume, is defined as exactly 10-3 m3. The milliliter is defined as 10-6 m3, or 1 cm3.
(a) Distinction between Mass and Weight
It is important to understand the difference between mass and weight. Mass is an invariant measure of the
amount 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 in USA at
1
, 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 t1attened 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=mxg
Where: w = weight of an object; m = mass; g = 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 ordinarily 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,
throughout this unit we will use mass rather than weight to describe the amounts of substances or objects. On
the other hand, for lack of a better word, "weigh" will be used for the act of determining the mass of an object.
Also, we will often say "weights" to mean the standard masses used in weighing.
(c)The Mole
2
Objectives
By the end of this topic the learner should be able to:
1. Explain the role of and apply the Avogadro's number in calculations in the study of chemistry.
2. Describe the methods used to compute the results of a quantitative analysis.
3. State the SI system of units in Chemistry and explain the distinction between mass and weight.
4. Discuss the mole as a measure of the amount of a chemical substance.
5. Express the concentrations of solutions in various ways.
6. Carry out calculations involving chemical stoichiometry.
1) Some Important Units of Measurement
(a) SI Units
Scientists throughout the world have adopted a standardized system of units known as the International
System of Units (SI). This system is based on the seven fundamental base units shown in the Table below.
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. For example:
The wavelength of yellow radiation used for determining sodium by flame photometry is about 5.9 x 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 x 10-6 L, or 50 µL
(microliters);
The amount of memory on some computer hard disks is about 20 x 109 bytes, or 20 Gbytes (gigabytes).
In analytical chemistry, we often determine the amount of chemical species from mass measurements. For
such 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), and sometimes microliters (µL).
The liter, the SI unit of volume, is defined as exactly 10-3 m3. The milliliter is defined as 10-6 m3, or 1 cm3.
(a) Distinction between Mass and Weight
It is important to understand the difference between mass and weight. Mass is an invariant measure of the
amount 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 in USA at
1
, 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 t1attened 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=mxg
Where: w = weight of an object; m = mass; g = 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 ordinarily 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,
throughout this unit we will use mass rather than weight to describe the amounts of substances or objects. On
the other hand, for lack of a better word, "weigh" will be used for the act of determining the mass of an object.
Also, we will often say "weights" to mean the standard masses used in weighing.
(c)The Mole
2
