Which of the following is the correct chemical formula for chromium(III) oxide?

• A. CrO3
• B. CrO2
• C. Cr2O3
• D. Cr3O2

Answer: (C)

Verified step by step guidance

1. Identify the oxidation state of chromium in chromium(III) oxide. The Roman numeral III indicates that chromium has a +3 charge, so the chromium ion is \( \text{Cr}^{3+} \).
2. Determine the charge of the oxide ion. Oxygen typically forms an oxide ion with a charge of \( \text{O}^{2-} \).
3. Balance the total positive and negative charges to form a neutral compound. Since chromium is \( +3 \) and oxide is \( -2 \), find the smallest whole number ratio of \( \text{Cr}^{3+} \) to \( \text{O}^{2-} \) that results in zero net charge.
4. Calculate the least common multiple of the charges: the LCM of 3 and 2 is 6. To get a total positive charge of +6, you need 2 chromium ions (2 \( \times \) +3 = +6). To get a total negative charge of -6, you need 3 oxide ions (3 \( \times \) -2 = -6).
5. Write the chemical formula using the ratio found: 2 chromium ions and 3 oxide ions combine to form \( \text{Cr}_2\text{O}_3 \), which is the correct formula for chromium(III) oxide.