Law of multiple proportions

In chemistry, the law of multiple proportions states that in compounds which contain two particular chemical elements, the amount of Element A per measure of Element B will differ across these compounds by ratios of small whole numbers. For instance, the ratio of the hydrogen content in methane and ethane per measure of carbon is 2:3. This law is also known as Dalton's Law, named after John Dalton, the chemist who first expressed it. The discovery of this pattern led Dalton to develop the modern theory of atoms, as it suggested that the elements combine with each other in multiples of a basic quantity.

The law of multiple proportions often does not apply when comparing very large molecules. For example, if one tried to demonstrate it using the hydrocarbons decane (chemical formula C₁₀H₂₂) and undecane (C₁₁H₂₄), one would find that 100 grams of carbon could react with 18.46 grams of hydrogen to produce decane or with 18.31 grams of hydrogen to produce undecane, for a ratio of hydrogen masses of 121:120, which is hardly a ratio of "small" whole numbers.