Final answer:
Joseph Louis Proust used basic copper carbonate to formulate the Law of Definite Proportions, demonstrating that a compound's constituent elements always have a constant composition by mass.
Step-by-step explanation:
The compound Joseph Louis Proust used to formulate the Law of Definite Proportions was basic copper carbonate. Proust conducted experiments showing that when a compound is broken down into its constituent elements, the masses of the constituents will always maintain the same proportions. This foundation led to the universal acceptance of the law, which posits that all samples of a pure compound contain the same elements in the same proportion by mass. For instance, sodium chloride will always have a set proportion of sodium to chlorine by mass. Similarly, sucrose will always maintain a specific percentage of carbon, hydrogen, and oxygen by mass.