If a question gives litres of gases at the same temperature and pressure, which law should be thought of first?

Correct answer: Law of gaseous volumes

When a chemistry question gives volumes of reacting gases in litres and specifies the same temperature and pressure, the first relevant idea is Gay-Lussac’s law of combining volumes. This law states that gaseous reactants and gaseous products combine in simple whole-number volume ratios when measured at the same temperature and pressure. For example, two volumes of hydrogen combine with one volume of oxygen to form two volumes of steam under suitable conditions. Option A is therefore the best first choice. Definite proportions concerns mass composition, reciprocal proportions compares elements, and conservation of mass concerns total mass.