text: Q:
  
  How do we compute voltage drop across a capacitor?
  
  When we have an OpAmp, for instance 
  
  Then, how could I compute how VO1 (AC + DC) changes when passing across C2?
  
  A:
  
  The problem with your circuit is that there is no dc bias point at U2's +input and every op-amp needs to have one. This is easily rectified with a resistor from +input to ground. This means that C2 and the new resistor (RN) form a high pass filter with a cut-off frequency of \$\frac{1}{2\pi R_N C_2}\$.
  
  