Atomic Mass The mass of an atom is primarily determined by the number of protons and neutrons in its nucleus. Atomic mass is measured in Atomic Mass Units (amu) which are

 

Fundamental Chemistry

C H A P T E R

Fundamental Chemistry 53

are made by weighing both the solute and the solvent and

the resultant solution may not have a volume of 100 mL.

Weight per Unit Volume (w/v)

This is a commoner method of preparing laboratory

solutions. In this method a weight of the solute is

dissolved in a final volume of 100 mL (it is wrong to

dissolve the weight of solute in 100 mL of solvent as is

often done).

A volumetric flask is the most accurate and convenient

container for preparing such solutions. Put the weighed

solute in the flask, add the solvent (keeping volume less

than the final volume required), dissolve the solvent

completely and then make up the total volume by adding

more of the solvent.

Wherever necessary, deduct the molecular weight of

water if the amount indicated is of anhydrous solute.

Volume per Unit Volume (v/v)

This is used when the ultimate solution is to be prepared

from liquids. A volume of the liquid solute is made up to

100 mL final volume with solvent. Here too, a volumetric

flask/beaker should be used and the final volume be made

up by adding the solvent. Care should be taken to work

under temperature conditions as indicated on the glass

equipment.

The type of percent solution should be ascertained

first, as a different type used instead of the required way

would cause substantial error ultimately, for instance,

a 50% w/w solution of sodium hydroxide is 75% w/v;

similarly, a 22½% w/v solution of sodium sulfate is only

10% w/w.

Part Dilutions

In this method, the dilution is expressed as that part of the

whole volume into which one part (or more parts) of the

solution is dissolved, e.g. WBC dilution is 1:20, implying

that there is one volume or part of blood in 20 volumes

or parts of final solution, though actually mixed with

19 volumes or parts of the diluting fluid.

Molar Solutions

A molar solution contains the molecular weight of the

solute (dissolved substance) in grams per liter of solution.

The molecular weight is found by adding the atomic

weights of the different atoms present in the compound

(Refer Periodic table).

Example: The molecular weight of sodium chloride is

58.454. Hence, 1 molar solution of the salt contains 58.454

grams of NaCl in 1 liter.

Formula to convert a percentage solution into a mol/L

solution:

g% (w/v) solution × 10

mol/L solution =

molecular weight of substance

To change a normal solution into a mol/L solution:

mol/L solution = Normality of solution

valence of substance

Normal Solutions

A normal solution is one which contains the gram equivalent

weight (equivalent weight in grams) of a substance per liter

of solution. The equivalent weight is the number of units of

the substance which will combine with or replace a single

unit of hydrogen, 35.5 units of chlorine, 8 units of oxygen,

etc. or the number of units of that substance which will

contain a single unit of hydrogen, 35.5 units of chlorine, 8

units of oxygen, etc.

1. The equivalent weight of an element is calculated by

dividing the atomic weight by the valency (valency

is the number of atoms of hydrogen one atom of the

element will combine with or displace).

 Example: The atomic weight of sulfur is 32.006 and its

valency is 2, the equivalent weight would be 32.006/2

= 16.003.

2. The equivalent weight of an acid is the weight of it in

grams which contains 1.008 g (one atomic weight)

of replaceable hydrogen. It is calculated by dividing

the molecular weight by the number of replaceable

hydrogen atoms in the molecule.

 Example: The molecular weight of sulfuric acid (H2SO4)

is 98.082 and the number of replaceable hydrogen

atoms is 2, hence its equivalent weight would be

98.082/2 = 49.041.

3. Equivalent weight of an alkali is that weight of it which

will neutralize the equivalent weight of an acid. It is

calculated by dividing the molecular weight by the

number of OH-

(hydroxyl) radicals in the molecule.

 Example: Calcium hydroxide; Ca (OH)2, has a

molecular weight of 91.014 and 2 hydroxyl groups, its

equivalent weight would be 91.014/2 = 45.507.

4. Equivalent weight of a♥ salt is calculated by dividing

its molecular weight by the number of metal ions

(cations) per molecule, multiplied by the valence of

the ion (cation).

 Example: Sodium sulfate has a molecular weight of

142.060, has 2 cations and the valency of the cation is

one, hence its equivalent weight would be:

 142.060 1

2

× = 71.030

54 Concise Book of Medical Laboratory Technology: Methods and Interpretations Saturated Solution

In this, the weight of the solute is not specified nor is the

volume of the solvent. It states that it contains as much as

will dissolve.

Standard Solution

The exact strength of standard solution is known and is

used for comparing strengths of other similar solutions.

PERIODIC TABLE OF ELEMENTS

Abbreviations and Definition

No. – Atomic Number

MP – Melting point

BP – Boiling point

¾ Density of elements with boiling points below 0°C is

given in g/L

¾ Earth crust composition average values are from a report

by FW Clarke and HS Washington, 1924. Elemental

composition of crustal rocks differ between different

localities

¾ Group: There are only 18 groups in the periodic table

that constitute the columns of the table. Lanthanoids

and Actinoids are numbered as 101 and 102 to separate

them in sorting by group.

Atomic Number

The number of protons in an atom each element is

uniquely defined by its atomic number.

Atomic Mass

The mass of an atom is primarily determined by the

number of protons and neutrons in its nucleus. Atomic

mass is measured in Atomic Mass Units (amu) which are

scaled relative to carbon, 12C, that is taken as a standard

element with an atomic mass of 12. This isotope of carbon

has 6 protons and 6 neutrons. Thus, each proton and

neutron has a mass of about 1 amu.

Isotope

Atoms of the same element with the same atomic number,

but different number of neutrons. Isotope of an element is