Transcript Molecular Formulas & Atomic Weights

Molecular Formulas &
Relative Atomic Mass
The Use of Avogadro’s
Hypothesis
Avogadro’s Hypothesis (1811)

Equal volumes of gases contain equal
numbers of particles.
Water’s Formula

Two volumes of hydrogen combine with
precisely one of oxygen.

This meant water’s molecular formula was H2O.
Molecular formula – how elements combine and
in what ratios to form compounds.

The Mole
Mole = an amount of a substance
 Another way to measure atoms
 2 moles of H combine with 1 mole of O to
form 1 mole of water

Relative Atomic Mass

Avogadro’s hypothesis also provided a way of
determining relative atomic masses.

H atom – relative mass of 1
O atom – relative mass of 16
O atoms weighed 16 times that of H atoms.



By 1860, a list of 60-some elements had
been developed including their…
 Properties
 Relative
atomic masses
 Known molecular formulas that they formed

It was found that…
 Oxygen
usually combined with 2 atoms of
another element
 Nitrogen with 3
 Carbon with 4
The Periodic Table
Dmitri Mendeleev tried to make sense of
this list & organize all the elements into a
useful table.
 But, it was no easy task.
