Chemistry · Some Basic Concepts Of Chemistry · NEET
It says that when gases react or are formed in a reaction, the volumes they use and make are in a simple whole-number ratio, as long as all gases are at the same temperature and pressure. Example from NCERT: 100 mL of hydrogen combines with 50 mL of oxygen to give 100 mL of water vapour. The ratio 100 : 50 : 100 is just 2 : 1 : 2. This matters for NEET because many gas questions can be solved just by using these volume ratios.
Avogadro's law (1811) says that equal volumes of all gases, at the same temperature and pressure, contain an equal number of molecules. So if two gas samples take the same volume in the same conditions, they have the same number of molecules, even if they are different gases. This is the reason we can compare gas volumes directly instead of counting molecules.
Gay-Lussac's law is an observation: gas volumes in a reaction follow simple whole-number ratios. Avogadro's law is the explanation: equal volumes have equal molecules, so those volume ratios are really molecule (mole) ratios. In short, Gay-Lussac tells you WHAT happens, and Avogadro tells you WHY it happens.
Because of Avogadro's law. If equal volumes hold equal molecules, then the ratio of volumes of gases equals the ratio of their molecules, which equals the ratio of their moles (at the same temperature and pressure). So for a gas-only reaction you can plug volumes straight into the balanced equation without converting to moles. This shortcut only works for gases at the same T and P.
Dalton's atomic theory explained the law of conservation of mass, the law of definite (constant) proportions, and the law of multiple proportions. It could NOT explain Gay-Lussac's law of gaseous volumes. Avogadro's law later explained it, by saying equal volumes of gases have equal numbers of molecules and that gases like hydrogen and oxygen are diatomic. This is a very common NEET one-liner question.
No. Avogadro's law is only for gases. Liquids and solids are packed tightly, so equal volumes of two liquids or solids do NOT contain equal numbers of molecules. The equal-volume-equal-molecules idea works only because gas molecules are far apart and the container mostly holds empty space.
Dalton's atomic theory could not explain which of the following?
When 1 dm3 of CO2 gas is passed over hot coke, the volume of the gaseous mixture after complete reaction at STP becomes 1.4 dm3. The composition of the gaseous mixture at STP is
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Joseph Louis Gay-Lussac gave the law of gaseous volumes in 1808. He noticed that reacting or produced gases combine in simple volume ratios when the temperature and pressure are the same for all of them.
Amedeo Avogadro proposed his law in 1811. He was the one who clearly separated the ideas of atoms and molecules, which let him explain Gay-Lussac's results.
Equal volumes of all gases, at the same temperature and pressure, contain an equal number of molecules. This is a direct one-mark statement that NTA has asked.
Two volumes of hydrogen react with one volume of oxygen to give two volumes of water vapour. Using equal-volume-equal-molecules, this becomes 2 molecules H2 + 1 molecule O2 -> 2 molecules H2O, which balances only if hydrogen and oxygen are diatomic. This is exactly why we know H2 and O2 are diatomic.
No. In this chapter, Gay-Lussac's law means the law of combining gaseous VOLUMES (1808). The pressure-temperature relation (P proportional to T) is a separate gas law you meet later. For Some Basic Concepts of Chemistry, always use the volume version.
