An acidified solution of potassium chromate was layered with an equal volume of amyl alcohol. When it was shaken after the addition of $1 \,mL$ of $3 \% H _{2} O _{2}$, a blue alcohol layer was obtained. 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 $X$ ?