Which of the following is a correct set of oxidation numbers for the atoms in calcium dihydrogen phosphate, Ca(H2PO4)2?
A. Ca= +2, H=+1, P=+5, O= -2
B. Ca= +2, H=0, P=+7, O= -8
C. Ca= +2, H= -1, P=+4, O= -2
D. Ca^2+= +2, H2PO4- = -1
E. Ca = +2, H = +1, P=+3, O= -2
The correct answer and explanation is :
Let’s work through this carefully.
Correct Answer:
✅ A. Ca = +2, H = +1, P = +5, O = -2
Detailed Explanation (300+ words):
First, let’s understand what we are dealing with. The chemical formula given is Ca(H₂PO₄)₂ — calcium dihydrogen phosphate. This compound contains a calcium ion (Ca²⁺) and two dihydrogen phosphate ions (H₂PO₄⁻).
When finding oxidation numbers:
- The oxidation number of a single atom in an ion is the same as the ion’s charge.
- Calcium (Ca) is an alkaline earth metal (Group 2 of the periodic table), and it almost always forms a +2 cation. Thus, Ca = +2.
- Hydrogen (H) when bonded to nonmetals (like oxygen or phosphorus) generally has an oxidation number of +1.
- Oxygen (O) in most compounds has an oxidation number of -2.
Now, we need to find the oxidation number of phosphorus (P) in H₂PO₄⁻.
Set up an equation for the total charge of one dihydrogen phosphate ion (H₂PO₄⁻):
[
2(\text{oxidation number of H}) + 1(\text{oxidation number of P}) + 4(\text{oxidation number of O}) = -1
]
Substituting known values:
[
2(+1) + (P) + 4(-2) = -1
]
[
2 + P – 8 = -1
]
[
P – 6 = -1
]
[
P = +5
]
Thus, in the H₂PO₄⁻ ion:
- Hydrogen is +1.
- Phosphorus is +5.
- Oxygen is -2.
Putting it all together for Ca(H₂PO₄)₂:
- Calcium = +2,
- Hydrogen = +1,
- Phosphorus = +5,
- Oxygen = -2.
Thus, choice A is the correct set of oxidation numbers.
Why the others are incorrect:
- B: Hydrogen is never 0 in such a compound, and oxygen cannot have -8 oxidation state.
- C: Hydrogen being -1 doesn’t match typical bonding patterns here.
- D: It talks about ions instead of atoms individually — it’s partially correct for overall charges but not for oxidation numbers of each atom.
- E: Phosphorus is incorrectly listed as +3 — it is +5 in this structure.