Which one of the following pair of substances illustrates law of multiple proportions?

Options

A. `CO, CO_(2)`
B. `NaCl, NaBr`
C. `H_(2)O, D_(2)O`
D. `MgO, Mg(OH)_(2)`

Answer

To determine which pair of substances illustrates the law of multiple proportions, we need to analyze the given pairs and see if they meet the criteria of the law. The law of multiple proportions states that when two elements combine to form more than one compound, the ratios of the masses of the second element that combine with a fixed mass of the first element can be expressed in simple whole numbers.

### Step-by-Step Solution:

1. **Identify the pairs of substances**: The given pairs are CO and CO2, NaCl and NaBr, H2O and D2O, MgO and Mg(OH)2.

2. **Analyze CO and CO2**:
- Both compounds contain carbon (C) and oxygen (O).
- In CO (carbon monoxide), 1 gram of carbon combines with 1.33 grams of oxygen.
- In CO2 (carbon dioxide), 1 gram of carbon combines with 2.67 grams of oxygen.
- The ratio of oxygen in CO to CO2 for a fixed mass of carbon is 1.33:2.67, which simplifies to 1:2.
- This pair illustrates the law of multiple proportions.

3. **Analyze NaCl and NaBr**:
- NaCl (sodium chloride) and NaBr (sodium bromide) consist of sodium (Na) combined with different halogens (Cl and Br).
- Since they involve different elements (chlorine and bromine), they do not illustrate the law of multiple proportions.

4. **Analyze H2O and D2O**:
- H2O (water) contains hydrogen (H) and oxygen, while D2O (heavy water) contains deuterium (D, an isotope of hydrogen) and oxygen.
- Although they involve the same element (oxygen), they are different isotopes of hydrogen, which means they do not illustrate the law of multiple proportions.

5. **Analyze MgO and Mg(OH)2**:
- MgO (magnesium oxide) contains magnesium (Mg) and oxygen, while Mg(OH)2 (magnesium hydroxide) contains magnesium, oxygen, and hydrogen.
- Since they contain different elements (hydrogen in Mg(OH)2), they do not illustrate the law of multiple proportions.

6. **Conclusion**: The only pair that illustrates the law of multiple proportions is CO and CO2.

### Final Answer:
The pair of substances that illustrates the law of multiple proportions is **CO and CO2**.