An oxidation state of Cr in K2Cr2O7

Analyze the question: You are asked to find the oxidation state of chromium (Cr) in potassium dichromate, K₂Cr₂O₇.

Concept: In a neutral compound, the oxidation states of all atoms add up to zero. Use these known values:

• Group 1 metals such as K are always +1
• Oxygen is usually -2

Solution:

Let the oxidation state of Cr = x.

K₂Cr₂O₇:

• 2 K atoms: 2(+1) = +2
• 2 Cr atoms: 2(x) = 2x
• 7 O atoms: 7(-2) = -14

Sum = 0:

2 + 2x - 14 = 0 2x - 12 = 0 2x = 12 x = +6

Check: 2(+1) + 2(+6) + 7(-2) = 2 + 12 - 14 = 0 ✓

Answer: The oxidation state of Cr is +6, which is option B.

Tip: Chromium in both chromates (CrO₄²⁻) and dichromates (Cr₂O₇²⁻) is +6. Dichromate is a strong oxidizing agent because Cr⁶⁺ is easily reduced to Cr³⁺, and this is why the orange solution turns green in redox reactions. For any oxidation-state question, set up the equation with the total charge on the right side and solve for x.