We start by calculating the %O. Since we know %C and %H we can just subtract it from the total to get %O.
The koala dines exclusively on eucalyptus leaves. Its digestive system detoxifies the eucalyptus oil, a poison to other animals. The chief constituent in eucalyptus oil is a substance called eucalyptol, which contains 77.87% C, 11.76% H, and the remainder O.
A mass spectrum of eucalyptol shows a peak at about 154 amu. What is the molecular formula of the substance?