What it does
Is the water in rain the same as the water in the sea, once it is purified? Chemists around 1800 argued about this. Joseph Proust showed that a pure compound always contains the same elements in the same proportion by mass, wherever it comes from and however it is made. This is the law of constant proportions (or definite proportions).
This page is a virtual lab. Pick a compound and where the sample came from, choose its mass, and press Analyse. The page splits the sample into the mass of each element, as read on a balance that shows 0.01 g, and adds a row to the table. Do it for several sources and sizes: the masses change, the ratio does not. Each row also shows that the element masses add up to the sample, the law of conservation of mass.
The Class 11 level adds the law of multiple proportions (John Dalton): when two elements make more than one compound, such as CO and CO₂, the masses of oxygen that join 1 g of carbon are in a simple ratio, here 1 : 2.
It fits the Class 9 science chapter Atomic Foundations of Matter and the Class 11 chemistry chapter Some Basic Concepts of Chemistry. Writing the formulas is in formula writer, and turning masses into moles is in mole bridge.
How to use it
- Choose a compound and where it came from.
- Set the sample mass in grams.
- Press Analyse this sample. The source moves on to the next one, so pressing again gives a new source.
- Read the table and the summary under it. Clear table starts again.
- Switch to Class 11 to compare compounds of the same two elements.
A worked example
Analyse 18 g of water. The balance reads about 2.01 g of hydrogen and 15.99 g of oxygen, so H : O ≈ 1 : 7.94. Now analyse 50 g of sea water (distilled): about 5.60 g of hydrogen and 44.40 g of oxygen, again 1 : 7.94. With school rounded masses (H = 1, O = 16), water H₂O has 2 g of hydrogen to 16 g of oxygen, which is the 1 : 8 quoted in textbooks.
In Class 11 mode, choose C and O. One gram of carbon joins 1.332 g of oxygen in CO and 2.664 g in CO₂: a ratio of 1 : 2.
Tips
- The tiny differences between rows come from the balance (±0.01 g), not from the compound.
- To find how much of an element is in a sample, use its share: oxygen is 8 parts of 9 in water.
- Mixtures (like air or sea water before purifying) do not follow this law: their make-up can change.
Check yourself
Four fresh questions on mass ratios and one on multiple proportions. Nothing is saved.
FAQ
Is the ratio exactly 1 : 8 for water? Almost: with today’s atomic weights it is 1 : 7.94. The 1 : 8 comes from rounding hydrogen to 1 and oxygen to 16.
Is this the NCERT book? No. It covers the same topic with fibub’s own samples and questions. fibub is not part of NCERT.
Where the numbers come from
- Standard atomic weights: IUPAC Commission on Isotopic Abundances and Atomic Weights (as used across fibub's chemistry tools). School rounded masses (H = 1, C = 12, O = 16, ...) for the textbook ratios.
- The laws of constant and multiple proportions are standard chemistry. Samples, sources and questions were written for fibub; the balance readings are simulated.
Standard scientific values, such as constants, atomic weights and melting points, are facts found in every textbook; we name where ours were checked. If you are an author or publisher and want a value, a credit or a table changed or removed, tell us and we will act quickly.
Last updated 2026-11-09.