JEE Main 2023 · 6 April · morning · Numerical answer · +4 / −0

In ammonium - phosphomolybdate, the oxidation state of Mo is + ___________

Answer and solution

Answer 6

Solution

Chemistry ma’am · 4 steps

Write the full formula first, then let the charges add up to zero and solve for Mo.

  1. Write the formula. Ammonium phosphomolybdate is (NH₄)₃[PMo₁₂O₄₀], the yellow precipitate used to test for phosphate. It contains 3 ammonium ions, 1 phosphorus, 12 molybdenum and 40 oxygen.
  2. Known oxidation states. Each NH₄⁺ carries +1, phosphorus is +5 as in phosphate, and each oxygen is -2. Let the oxidation state of Mo be x. The compound is neutral, so the total is zero.

    3(+1) + (+5) + 12x + 40(-2) = 0

  3. Solve for x. 3 + 5 + 12x - 80 = 0, so 12x = 72 and x = +6. You get the same answer from the older way of writing it, (NH₄)₃PO₄ . 12MoO₃, where Mo in MoO₃ is +6.
  4. So. The oxidation state of Mo in ammonium phosphomolybdate is +6, so the answer is 6.

The idea to keep. For a big complex salt, remember its formula, fill in the oxidation states you already know, and set the total to the overall charge. Heteropoly acids like this always keep the metal in its highest oxidation state.

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

Study Redox Reactions

Asked about 90 times in JEE Main since 2020. This question is from 2023.