Antoine law and ideal gas law
Assume that the initial temperature is room temperature and neglect the initial amount of acetonitrile in the gas phase.  Calculate the equilibrium vapor pressure of acetonitrile at 140 C and compare it with the partial pressure you would calculate from the ideal gas law if all the acetonitrile had evaporated (so that its volume is 100 cc).  If the latter is greater than the former, then there will still be some acetonitrile remaining as liquid.  The partial pressure of the acetonitrile in the gas phase will then be equal to the equilibrium vapor pressure.  Assuming that x gm of acetonitrile have evaporated, you can then calculate, in terms of x, the new mass and volume of the liquid and the new volume of the head space.  You can then determine x from the requirement that the total amount of acetonitrile has not changed.