On heating a mixture of common salt and K2Cr2O7 in equal amount along with concentrated H2SO4 in a test tube, a gas is evolved. Formula of the gas evolved and oxidation state of the central metal atom in the gas respectively are:
- Cr2O2Cl2 and +3
- Cr2O2Cl2 and +6
- CrO2Cl2 and +6
- CrO2Cl2 and +5
Answer
(3) CrO2Cl2 and +6
Solution
This is the chromyl chloride test for chloride ion. Orange-red vapours of chromyl chloride are evolved:
4NaCl + K2Cr2O7 + 6H2SO4 2KHSO4 + 4NaHSO4 + 2CrO2Cl2 + 3H2O
Oxidation state of Cr in CrO2Cl2 (let it be x):
x + 2(-2) + 2(-1) = 0
x = +6
Gas = CrO2Cl2, oxidation state of Cr = +6
Answer: (3)