Taken from OCR A Chemistry Unified Chemistry Sample Paper.
Information about a monobasic organic acid D is shown below.
- D reacts by both electrophilic substitution and electrophilic addition.
- The molecular formula of D is $\ce{C_xH_yO2}$.
- The mass spectrum of D has a molecular ion peak at $m/z = 148$.
- The $\ce{^13C}$ NMR spectrum of D contains seven peaks.
Determine and draw a possible structure for D. Explain your reasoning from the evidence provided.
I understand that D must contain a benzene ring (electrophilic substitution) and a double carbon bond (electrophilic addition). However, I'm at a loss about how to deduce the molecular formula. Taking away $M_r(\ce{O2})$ from $\pu{148 g mol^-1}$, gives $M_r(\ce{C_xH_y})= \pu{116 g mol^-1}$. The mark scheme gives the molecular formula as $\ce{C9H8O2}$. I can't see how the values of $x$ and $y$ were deduced. All you know is that D contains a $\ce{C=C}$, benzene ring and most likely a $\ce{-COOH}$ group (due to monobasic nature).