Chemistry · Some Basic Concepts Of Chemistry · NEET
The coefficients are always a mole ratio. But for GASES at the same temperature (T) and pressure (P), volume is directly proportional to moles (this is Avogadro's Law: equal volumes have equal numbers of molecules). So the mole ratio and the volume ratio become the SAME numbers. That is why for N2 + 3H2 -> 2NH3, you can say 1 volume N2 + 3 volumes H2 -> 2 volumes NH3, using litres or dm3 straight from the coefficients.
No. That is the whole point of volume-volume stoichiometry. As long as every substance you use is a gas and they are all at the same T and P, you skip the mole step. Just plug the volumes into the coefficient ratio. Converting to moles gives the same answer but wastes time in the exam.
No. It only works for GASES measured at the same T and P. Solids and liquids do NOT follow the volume ratio (their volume is not proportional to moles). If a reaction has a solid like carbon (C) or CaCO3, you IGNORE its volume and only apply the ratio to the gaseous species.
When gases react (or are formed), the volumes of the reacting gases and the product gases are in a simple whole-number ratio, if measured at the same temperature and pressure. Example: 1 volume of H2 + 1 volume of Cl2 gives 2 volumes of HCl. NEET fact: Dalton's atomic theory could NOT explain this law; Avogadro's hypothesis explained it later.
Set up a variable. Let x be the volume of gas that reacts. Write how much reactant is left and how much product forms, using the coefficient ratio. For CO2 + C(s) -> 2CO: if x dm3 of CO2 reacts, CO2 left = (1 - x), CO formed = 2x. Add the gas volumes to get the total, set it equal to the given total, and solve for x. The solid carbon has no volume term.
Only the gases. In CaCO3(s) -> CaO(s) + CO2(g), the two solids have no meaningful gas volume, so you only track CO2. In CO2(g) + C(s) -> 2CO(g), you only put CO2 and CO into the volume ratio and leave C(s) out completely.
Because the number of gas molecules changes. In CO2 + C(s) -> 2CO, 1 volume of gas (CO2) becomes 2 volumes of gas (CO), so total gas volume increases. If you know the starting and final total volumes, you can find exactly how much reacted. This is a very common NEET question style.
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
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.
For gases at the same temperature and pressure, yes. Avogadro's Law makes volume proportional to moles, so both ratios are the same numbers. For solids and liquids, no; only the mole ratio works there.
All the species you compare must be gases, and they must be at the same temperature and pressure. Then you can use the balanced equation coefficients directly as a volume ratio in litres or dm3.
Usually no. Because you only need the RATIO of volumes, the 22.4 L (or 22.7 L) molar volume cancels out. You only need molar volume if a problem mixes gas volume with mass or number of molecules.
H2(g) + Cl2(g) -> 2HCl(g): 1 volume of hydrogen plus 1 volume of chlorine gives 2 volumes of hydrogen chloride, all at the same T and P. The coefficients 1, 1, 2 are the volume ratio.
NEET regularly asks fast questions where converting to moles wastes time. Using volume ratios directly (like the CO2-over-coke problem) lets you solve in seconds, and the theory link to Dalton and Avogadro is a common one-mark fact.
