Laws of Definite Proportions and Multiple Proportions
One of Dalton’s postulates, written in 1808, was that atoms combine in simple
whole-number ratios to form compounds.
This ratios are represented in our familiar
formulas today, such as H
This was suggested to Dalton by the
Law of Definite Proportions, which states that all compounds have a definite percent by
mass of each element making it up.
For example, 100 g of carbon dioxide always
contains 27.3 g of carbon (27.3% carbon by mass), regardless of the source of the carbon
In the same way, 100 g of water always contains 88.9 g of oxygen (88.9%
oxygen by mass).
This suggested to Dalton that each compound had a definite ratio, in
terms of simple whole numbers, of the atoms making it up—what we call a formula.
Dalton also had information about compounds which contain the same elements
but with different mass percentages.
Say we had compound X (known today as carbon
dioxide) which had 27.3% by mass carbon, meaning that in 100 g total, there are 27.3 g
carbon and 72.7 g oxygen.
Another compound Y also contains carbon and oxygen, and
100 g of that compound contains
42.9 g carbon and 57.1 g oxygen.
This means that
compound X has
or 2.66 times more O than C (by mass), and compound Y has
= 1.33 times more O than C (by mass).
Summarizing these result in a table:
This table shows that for a fixed 1 unit of mass of carbon, compound X has twice as
much O than compound Y.
This means that if compound X is CO
, compound Y must
Or if compound Y is CO
, compound X is CO
Dalton didn’t have a table of
atomic masses in 1808—these were developed gradually over the 19
century, but data
such as this indicates that there are definite relationships between two related
compounds, and further suggested to Dalton and others, the idea that compounds have
definite formulas with atoms in simple whole number ratios.
The data illustrated in the CO, CO
example is an illustration of the Law of
If two elements combine to form more than compound, the masses of one of these
elements that combine with a fixed mass of the other, is in a simple whole number ratio.