An acidified solution of potassium chromate was layered with an equal volume of amyl alcohol. When it was shaken after the addition of 1ml of 3% `H_2O_2,` a blue alcohol layer was obtianed. The blue color is due to the formation of a chromium (VI) compound 'X'. What is the number of oxygen atoms bonded to chromium through only single bonds in a molecule of 'X' ?