mass
'no magic to the mole'
amount
molar mass
concentration
solution volume
gas volume
molar gas volume
Avogadro
constant, L
number of
entities, N
Now try the following question.
C11. CALCULATING EMPIRICAL & MOLECULAR FORMULAE FROM DATA
Analytical chemistry approaches both qualitative and quantitative problems. Qualitative analysis ascertains the identity of the components or/& constituents in a sample. Quantitative analysis targets how much of each component or constituent is present in the sample. Analytical chemistry has a direct impact in clinical analysis, pharmaceutical research, quality control and environmental analysis, for instance in such basic everyday necessities as drinking water so that it is, and remains, potable.
In chemical analysis frequently it is the elemental composition of a substance that is established first of all. Nowadays, with the advances that have taken place over the last 50 years or so, in instrumentation and its interfacing with computers – particularly for spectroscopic methods – more information than a substance’s empirical formula may be available routinely. Nevertheless, empirical formulae do occupy a special place in chemical analysis historically and, in any event, chemical formulae are established experimentally first of all.
11.1 INFORMATION PROVIDED BY DIFFERENT TYPES OF FORMULAE
(a) MOLECULAR FORMULA
– gives the atomic symbols of the atoms present in one molecule of a substance with the number(s) of these atoms being implied by integer sub-scripts in the written formula.
Experimental data required to calculate a molecular formula are:
(i) composition of the substance quoted either as mass % or masses;
& (ii) molar mass of the substance, M, or its corresponding Mr value.
MOLECULAR FORMULA
C2H6
Suppose as above right that we have a compound with a molecular formula C2H6. Historically, chemical analysis would have yielded initially only the simplest ratio and identity of the combining atoms. Formulae reporting only that basic level of information are referred to as empirical formulae.
(b) EMPIRICAL FORMULA
– gives the chemical symbols of the atoms present in a compound and implies the simplest ratio between the numbers of these atoms by integer sub-scripts in the written formula.
Experimental data required to calculate an empirical formula are:
(i) composition of the compound quoted either as mass % or masses;
& (ii) molar masses of constituent elements or corresponding Ar values.
EMPIRICAL FORMULA
CH3
A molecular formula and an empirical formula can be indistinguishable in more than a few simple cases, especially where an odd number exists in the molecular formula of a covalent compound for at least one of the constituent atoms,
For ionic compounds, by convention, the empirical formula is reported, e.g., aluminium fluoride, AℓF3. Additional structural information is required to gauge co-ordination numbers for the ions in the lattice.
To determine the empirical formula of a compound from data, the chemical amount of each element present in
a sample must be calculated. Thence we determine the simplest whole-number ratio of the chemical amounts. Effectively, two or more n = m / M type calculations are being carried out side by side.
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