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AQA GCSE · Question 07.4 · Bonding structure and the properties of matter
The most common oxides of iron are Fe₂O₃ and Fe₃O₄.
What is the ratio of the numbers of ions in Fe₃O₄?
The most common oxides of iron are Fe₂O₃ and Fe₃O₄.
What is the ratio of the numbers of ions in Fe₃O₄?
Answer options:
A.
2 Fe²⁺ : 1 Fe³⁺ : 4 O²⁻
B.
1 Fe²⁺ : 2 Fe³⁺ : 4 O²⁻
C.
3 Fe²⁺ : 4 O²⁻
D.
3 Fe³⁺ : 4 O²⁻
How to approach this question
1. The formula is Fe₃O₄. This means there are 3 iron ions and 4 oxide ions.
2. The oxide ion always has a charge of 2- (O²⁻).
3. Calculate the total negative charge from the 4 oxide ions: 4 × (-2) = -8.
4. For the compound to be neutral, the total positive charge from the 3 iron ions must be +8.
5. Iron can form Fe²⁺ and Fe³⁺ ions. You need to find a combination of three iron ions that adds up to a total charge of +8.
6. Test the options:
- A: (2 × +2) + (1 × +3) = +4 + +3 = +7 (Incorrect)
- B: (1 × +2) + (2 × +3) = +2 + +6 = +8 (Correct)
- C: (3 × +2) = +6 (Incorrect)
- D: (3 × +3) = +9 (Incorrect)
Full Answer
B.1 Fe²⁺ : 2 Fe³⁺ : 4 O²⁻✓ Correct
1 Fe²⁺ : 2 Fe³⁺ : 4 O²⁻
The compound Fe₃O₄ is a mixed oxide, sometimes written as FeO·Fe₂O₃. It contains iron in two different oxidation states (+2 and +3).
To determine the ratio of ions, we must ensure the overall compound is electrically neutral.
1. The formula shows there are 4 oxide ions. The charge on an oxide ion is always 2-.
Total negative charge = 4 × (-2) = -8.
2. The formula shows there are 3 iron ions. The total positive charge from these 3 ions must balance the negative charge, so it must be +8.
3. We need to find a combination of Fe²⁺ and Fe³⁺ ions that gives 3 ions in total with a total charge of +8. Let's check the given option B:
- One Fe²⁺ ion has a charge of +2.
- Two Fe³⁺ ions have a total charge of 2 × (+3) = +6.
- Total positive charge = (+2) + (+6) = +8.
4. This combination gives a total of 1 + 2 = 3 iron ions and a total charge of +8, which correctly balances the -8 charge from the four oxide ions.
Therefore, the correct ratio is 1 Fe²⁺ : 2 Fe³⁺ : 4 O²⁻.
Common mistakes
✗ Not knowing the charge of the oxide ion is 2-.
✗ Making an arithmetic error when calculating the total positive charge for each option.
✗ Assuming all iron ions have the same charge.
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