The oxidation states of transition metal atoms in K₂Cr₂O₇, KMnO₄ and K₂FeO₄, respectively, are x, y and z. The sum of x, y and z is _______.

Solution

Chemistry ma’am · 3 steps

Potassium is +1 and oxygen is −2, so each neutral salt tells you its metal's oxidation state.

1. Set the rule. In a neutral compound all oxidation states add up to zero. Take K as +1 and O as -2, then solve for the metal.

   Compound	Charge balance	Metal's oxidation state
   K₂Cr₂O₇	2(+1) + 2x + 7(-2) = 0, so 2x = 12	x = +6 (per Cr atom)
   KMnO₄	(+1) + y + 4(-2) = 0	y = +7
   K₂FeO₄	2(+1) + z + 4(-2) = 0	z = +6

2. Two chromium atoms. In dichromate the +12 is shared by two Cr atoms. The oxidation state belongs to one atom, so x = 6, not 12.
3. So. x + y + z = 6 + 7 + 6 = 19.

The common slip: 25. Using the combined charge of both Cr atoms in dichromate, 12 instead of 6, gives 12 + 7 + 6 = 25, not 19.

The idea to keep. Oxidation state is always for one atom. When a formula has two metal atoms, find their combined charge first, then divide by two.

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