Final answer:
Methane and ethane demonstrate the law of definite composition by always having a consistent composition of elements in a specific proportion by mass.
Step-by-step explanation:
Methane and ethane both demonstrate the law of definite composition in chemistry. This law states that a given compound always contains the same elements in the same proportion by mass.
Methane, which is composed of one carbon atom and four hydrogen atoms, always has a specific ratio of carbon to hydrogen. This ratio is 1:4 by mass, meaning that for every 12 grams of carbon, there are 4 grams of hydrogen in methane.
Similarly, ethane is composed of two carbon atoms and six hydrogen atoms. The ratio of carbon to hydrogen in ethane is always 2:6 by mass, which simplifies to 1:3. Therefore, ethane always has a consistent composition of 1 part carbon to 3 parts hydrogen by mass.