Which pair of substances could be used to illustrate the law of multiple proportions?

• A. O_2 and O_3
• B. CO and CO_2
• C. H_2O and H_2O_2
• D. NaCl and KCl

Answer: (B)

Verified step by step guidance

1. Understand the law of multiple proportions: it states that when two elements form more than one compound, the masses of one element that combine with a fixed mass of the other are in ratios of small whole numbers.
2. Identify pairs of substances that contain the same two elements but differ in the ratio of those elements in their formulas.
3. Analyze O_2 and O_3: both are forms of oxygen only, so they do not illustrate the law because they contain only one element.
4. Analyze CO and CO_2: both contain carbon and oxygen, but in different ratios (1:1 and 1:2), which fits the law of multiple proportions.
5. Analyze H_2O and H_2O_2: both contain hydrogen and oxygen, but the ratio of oxygen to hydrogen is not a simple whole number ratio relative to a fixed mass of hydrogen, so this pair also illustrates the law but is less straightforward than CO and CO_2.