Chemistry · Some Basic Concepts Of Chemistry · NEET
If the SAME two elements can combine to make more than one compound, then fix the mass of one element. Now look at the masses of the second element in each compound. These masses will always be in a ratio of small whole numbers like 1:2, 2:3, or 1:3. It matters for NEET because it proves atoms combine in fixed, countable units (whole atoms), which supports Dalton's atomic theory.
Definite proportions is about ONE compound: a single compound always has the same elements in the same fixed mass ratio, no matter the source (water is always 1:8 H:O). Multiple proportions is about TWO OR MORE compounds of the same elements: it compares them and finds a small whole number ratio. So: definite = one compound stays constant; multiple = compare several compounds.
Step 1: find the molar mass of each compound using molar mass = given mass divided by moles. Step 2: write X + 2Y = (mass of XY2) and 3X + 2Y = (mass of X3Y2). Step 3: subtract the first equation from the second. The Y terms cancel, giving 2X = difference, so you get X. Step 4: put X back to find Y. This is just solving two simple equations.
Both compounds contain 2 atoms of Y (Y2). When you subtract X + 2Y from 3X + 2Y, the 2Y cancels out completely. You are left with only X terms: 3X minus X = 2X. This trick removes Y so you can find X first. This is why the XY2 / X3Y2 pair is a favourite NEET question.
Yes, that is the whole point. After you fix the mass of one element, the masses of the other element must reduce to a SIMPLE whole number ratio (like 1:2, not 1.37:2.6). If you get an ugly non-whole ratio, you made a calculation mistake. Small whole numbers are the signature of this law.
John Dalton proposed it in 1803. Do not confuse it with Gay-Lussac's law of gaseous volumes (1808) or Avogadro's law (1811). NEET often mixes these names in one question, so remember: Multiple proportions = Dalton = 1803.
Suppose the elements X and Y combine to form two compounds XY2 and X3Y2. When 0.1 mole of XY2 weighs 10 g and 0.05 mole of X3Y2 weighs 9 g, the atomic weights of X and Y respectively are:
Dalton's atomic theory could NOT explain which of the following?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Hydrogen and oxygen form water and hydrogen peroxide. Fixing hydrogen at 2 g, water uses 16 g oxygen and hydrogen peroxide uses 32 g oxygen. The oxygen ratio 16:32 = 1:2, a small whole number ratio. Carbon and oxygen (CO and CO2) is another common example, with an oxygen ratio of 1:2.
No. The mole concept counts particles (6.022 x 10^23 per mole). The law of multiple proportions is one of the five basic laws of chemical combination and is about mass ratios between different compounds of the same two elements. They are different topics but often appear in the same NEET chapter.
Always a simple whole number ratio such as 1:1, 1:2, 2:3, or 1:3. If your ratio does not simplify to small whole numbers, recheck your arithmetic. Non-simple ratios are a sign of a mistake, not a valid answer.
After multiple proportions, study Gay-Lussac's law of gaseous volumes and Avogadro's law. These deal with gas volumes instead of masses and explain what Dalton's theory could not, so NEET frequently links them together in single questions.
