Page 288 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

Element R.A.M.

Hydrogen
Sulphur
Oxygen
Relative Molecular Mass = 98.0744

This approach can be used to obtain the R.A.M. of any compound, so that
1 mole of Hg,Cl, has a mass of 0.472 09 kg
1 mole of Na,CO,, 10H20 has a mass of 0.286 141 kg
1 mole of H, SO, has a mass of 0.098 074 kg
It follows from this, that a molar solution of sulphuric acid will contain
98.074 grams of sulphuric acid in 1 litre of solution, or 49.037 grams in 500 mL
of solution. Similarly, a 0.1 M solution will contain 9.8074 grams of sulphuric
acid in 1 litre of solution, and a 0.01 M solution will have 0.980 74 gram in the
same volume. So that the concentration of any solution can be expressed in
terms of the molar concentration so long as the weight of substance in any
specified volume is known.

10.4 EQUIVALENTS, NORMALITIES AND OXlDATlON NUMBERS
Although molar concentrations are now commonly used in determinations of
reacting quantities in titrimetric analysis, it has been traditional to employ other
concepts involving what are known as 'equivalent weights' and 'normalities'
for this purpose. In neutralisation reactions the equivalent weight/normality
concept is relatively straightforward, but for reduction-oxidation titrations it
often requires an understanding of what are known as 'oxidation numbers' of
the substances involved in the redox reaction. Although the modern approach
is to discard this form of calculation and quantitation, the authors of this book
fully appreciate that there are many scientists who do prefer to use it, and some
who claim it has clear advantages over the molar concept. Because of this, a
full explanation of this approach to titrimetry is retained as Appendix 17 but
al1 other quantitative aspects in this book are in terms of moles per litre.

10.5 PREPARATION OF STANDARD SOLUTIONS
If a reagent is available in the pure state, a solution of definite molar strength
is prepared simply by weighing out a mole, or a definite fraction or multiple
thereof, dissolving it in an appropriate solvent, usually water, and making up
the solution to a known volume. It is not essential to weigh out exactly a mole
(or a multiple or sub-multiple thereof); in practice it is more convenient to
prepare the solution a little more concentrated than is ultimately required, and
then to dilute it with distilled water until the desired molar strength is obtained.
If M, is the required molarity, VI the volume after dilution, M, the molarity
originally obtained, and V, the original volume taken, Ml VI = M, V,, or
VI = M, V2/Ml. The volume of water to be added to the volume V, is
( VI - V,) mL.
The following is a list of some of the substances which can be obtained in a