Single correct · +4 / −1

The oxidation state of phosphorus in H₃PO₄ is:

  1. (a) +3
  2. (b) +5
  3. (c) +1
  4. (d) +4
Answer and solution

Answer (b) +5

Solution

Chemistry ma’am · 3 steps

Let phosphorus be x, give every H and O its usual number, and make the whole molecule add up to zero.

  1. Fixed values. In H₃PO₄, hydrogen bonded to oxygen is +1 and oxygen is -2. The molecule is neutral, so all the oxidation states must add up to 0.

    3(+1) + x + 4(-2) = 0

  2. Solve for x. 3 + x - 8 = 0, so x = +5. This fits: phosphorus has 5 valence electrons, and +5 is the highest state it can reach.
  3. So. Phosphorus is in the +5 state in H₃PO₄. The answer is (b).

The idea to keep. For any oxyacid, fix H at +1 and O at -2, then solve so the sum is zero. A central atom can never go above its group's maximum, which is +5 for phosphorus.

Answer from the NTA final answer key. Solution by PrepWiser.

Study Redox Reactions

Asked about 90 times in JEE Main since 2020.