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Every stoichiometry problem you solve quietly assumes one thing: that water is always 11% hydrogen by mass, wherever it comes from, and that copper carbonate always has the same share of copper. Around 1800 that was still being argued. This post tells how Joseph Proust settled it, works through the numbers the way your textbook does, and ends with what you can take from his method.
Who was Joseph Proust?
Your NCERT Class 11 chemistry chapter, Some Basic Concepts of Chemistry, introduces him as a French chemist who lived from 1754 to 1826 and gave the law of definite proportions. Most of his productive career was spent in Spain, according to a history in the Revue d'histoire de la pharmacie (1999), which also notes that he had trained, without finishing, as an apothecary, and that this showed in his careful work as an analyst.
An article in Platinum Metals Review (1993) describes him as Professor of Chemistry and Metallurgy at the Royal Artillery School in Segovia, working at the Segovian House of Chemistry in the late eighteenth century and then at a platinum laboratory in Madrid. It credits him with "a systematic approach" and "great attention to detail", even in work where he interpreted his results wrongly.
What does the law of definite proportions say?
In NCERT's words, "a given compound always contains exactly the same proportion of elements by weight." The book's evidence is Proust's study of two samples of cupric carbonate, one natural and one made in the laboratory. Both gave the same result: 51.35% copper, 9.74% carbon and 38.91% oxygen. His conclusion was that, whatever the source, a compound contains the same elements combined in the same proportion by mass. The law is also called the law of definite composition.
Worked example: checking copper carbonate yourself
Take CuCO₃ with today's atomic masses: Cu = 63.55, C = 12.01, O = 16.00.
Molar mass: 63.55 + 12.01 + 3 × 16.00 = 123.56 g/mol.
Copper: 63.55 / 123.56 × 100 = 51.43%.
Carbon: 12.01 / 123.56 × 100 = 9.72%.
Oxygen: 48.00 / 123.56 × 100 = 38.85%.
Check: 51.43 + 9.72 + 38.85 = 100.00%.
These are close to, but not exactly, the figures NCERT quotes, which is a reminder that measured values carry experimental error. The law does not claim your numbers match a textbook to the last digit. It claims that two pure samples of the same compound match each other.
Now use it. A 5.00 g sample of copper carbonate, from any source, contains 5.00 × 0.5143 = 2.57 g of copper. If a sample gave 3.10 g, you would conclude it was not pure copper carbonate. For more practice going from percentages to formulas, see our post on empirical and molecular formula problems.
What was the Berthollet-Proust controversy?
The chemist Berthollet disagreed. Writing in Nature in 1894, Philip Hartog of Owens College, Manchester, argued that textbooks had misstated the dispute. Berthollet, he wrote, thought that in most cases two substances could combine "in an infinite number of ratios varying continuously between certain limits". Proust held that chemical combination happens in a single ratio or a series of separate ratios.
Hartog's point is a useful one for students. Both men were partly describing different things: true compounds, and what we now call solutions and other homogeneous mixtures. Proust's view won, he wrote, largely because of the weight of John Dalton's law of multiple proportions and atomic theory. NCERT dates that law to 1803. Our post on Dalton's atomic theory picks up the story.
There is a modern footnote too. A short paper in the Journal of Chemical Education (1975) is titled "The failings of the law of definite proportions", and its abstract says simply: "Inorganic solids often violate the law of definite proportions." For school and entrance exams, use the law as stated; just know that some solids are exceptions.
Common mistakes with the laws of chemical combination
Mixing up definite and multiple proportions. Definite: one compound, fixed ratio. Multiple: two elements forming more than one compound, such as CO and CO₂, where the masses of oxygen combining with a fixed mass of carbon are in a simple ratio, here 1:2.
Treating a mixture as a compound. Air or salt water can vary in composition; that does not break the law, because they are not compounds.
Rounding too early. Keep four significant figures until the last step, or your percentages will not add to 100.
If a step like converting percentages to moles keeps going wrong, the Fermi Problem Library has a step-by-step solver that explains the reasoning rather than printing an answer, so you can find the exact line where your method slipped.
What can a student learn from Proust's way of working?
Proust's case rested on comparing samples from different sources and showing they matched. That is a habit worth copying. When you learn a rule, test it on two examples that look different. If the rule holds for both, you understand it; if it does not, you have found something worth asking about.
Today, calculate the mass percentages in CO and CO₂, then explain in one sentence which law each pair of numbers illustrates.
