Which one of the following statements about the law of conservation of mass is correct?
- (a)A given compound always contains exactly same proportion of elements.
- (b)When gases combine in a reaction, they do so in a simple ratio by volume, provided all gases are at room temperature.
- (c)Matter can neither be created nor destroyed.
- (d)Equal volumes of all gases at same temperature and pressure contain equal number of molecules.
Correct — C, Matter can neither be created nor destroyed. That sentence is the law of conservation of mass in its usual textbook form, and its practical meaning is that in any chemical reaction the total mass of the products equals the total mass of the reactants. Atoms are only rearranged; none appears from nowhere and none vanishes. It is the reason a chemical equation has to be balanced, with the same number of atoms of each element on both sides. The other three statements are all genuine laws of chemical combination, but each belongs to a different one, and the question is really asking whether a candidate can attach the right words to the right name.
- (a)A given compound always contains exactly same proportion of elements. — This is the law of definite or constant proportions, associated with Proust. It says that pure water is always eight parts oxygen to one part hydrogen by mass, wherever it comes from — a statement about composition, not about mass being conserved during a reaction.
- (b)When gases combine in a reaction, they do so in a simple ratio by volume, provided all gases are at room temperature. — This is Gay-Lussac's law of gaseous volumes. It is also imprecisely worded here: the ratio holds when the volumes are compared at the same temperature and pressure, not specifically at room temperature.
- (d)Equal volumes of all gases at same temperature and pressure contain equal number of molecules. — This is Avogadro's law. It was the idea that resolved the confusion in Gay-Lussac's volume ratios by separating atoms from molecules, but it says nothing about the conservation of mass.
The laws of chemical combination are the empirical foundation on which Dalton built the atomic theory. Conservation of mass says nothing is gained or lost in a reaction. Constant proportions says a compound's composition is fixed. Multiple proportions says that when two elements form more than one compound, the masses of one that combine with a fixed mass of the other are in a small whole-number ratio. Gay-Lussac's law of gaseous volumes and Avogadro's law then extend the same whole-number thinking to gases.
Four laws, four sentences, one label — the question is testing labels, so learn them as a set rather than one at a time. A memory hook that works is to notice what each sentence is about. Conservation is about mass before and after; definite proportions is about the make-up of one compound; Gay-Lussac is about volumes of gases combining; Avogadro is about how many molecules a volume holds. Only the third of these is about mass at all, which points straight at option (c). It is also worth knowing that the law is credited to Antoine Lavoisier, whose careful weighing of sealed reaction vessels in the 1770s showed that combustion added mass from the air rather than losing a substance called phlogiston.
- The law of conservation of mass means the total mass of the products of a chemical reaction equals the total mass of the reactants.
- It is the reason every chemical equation must be balanced, atom for atom, on both sides.
- The law of definite proportions states that a pure compound always contains the same elements in the same fixed proportion by mass.
- Avogadro's law states that equal volumes of all gases, at the same temperature and pressure, contain equal numbers of molecules.
- Confusing the law of definite proportions with the law of multiple proportions, which is about two elements forming several compounds.
- Reading option (b) as Avogadro's law because it mentions gases; Gay-Lussac's law is about combining volumes, Avogadro's about the number of molecules.
- Extending conservation of mass to nuclear reactions, where mass and energy are interconvertible and only the two together are conserved.
Asked as a definition-matching item where all four statements are true laws and only one carries the name in the stem.
The proposition ‘equal volumes of different gases contain equal numbers of molecules at the same temperature and pressure’ is known as
- (a) Avogadro’s hypothesis
- (b) Gay-Lussac’s hypothesis
- (c) Planck’s hypothesis
- (d) Kirchhoff’s theory
Answer(a) Avogadro’s hypothesis
Puts the correct name on the statement that appears here as option (d). Anyone who has met that item will not mistake Avogadro's law for the law of conservation of mass.
Equal volume of all gases, when measured at the same temperature and pressure, contain an equal number of particles. Who proposed the above law?
- (a) Charles
- (b) Boyle
- (c) Avogadro
- (d) Lussac
Answer(c) Avogadro
The same law again, this time asked for its author, with Gay-Lussac planted as the near miss — exactly the pair of laws that options (b) and (d) here are built from.
- practice — not a real PYQ
The statement that a pure chemical compound always contains the same elements combined in the same proportion by mass is known as the law of
- (a)conservation of mass
- (b)definite proportions
- (c)multiple proportions
- (d)reciprocal proportions
Answer(b) definite proportions — Proust's law, which fixes a compound's composition regardless of how or where it was prepared.
- practice — not a real PYQ
A chemical equation has to be balanced because of the law of
- (a)conservation of mass
- (b)constant proportions
- (c)gaseous volumes
- (d)partial pressures
Answer(a) conservation of mass — atoms are only rearranged in a reaction, so the same number of each kind must appear on both sides.