Chemical sensors are ubiquitous in the world, with important and broad applications in medicine, industrial process monitoring, automotive and aerospace applications, and the military. Due to its unique electrical properties and an ideal two-dimensional structure, graphene shows great promise as a highly sensitive, low noise sensor material. In this work, we report the results of comprehensive first-principles simulation and modeling of the electrostatic response of graphene when exposed to multiple chemical vapors, including hexane, tetrahydrofuran, toluene, and acetone. The effects of different graphene surface defects, such as vacancy defects, hydrogen defects, oxygen defects and hydroxyl group defects on graphene conduction have been explored, including how these defects affect the response of graphene when exposed to each of the studied chemical vapors. The simulation results were analyzed and compared with experimental data with a focus on understanding the physics and kinetics of the adsorption and desorption of gas molecules on the graphene surface. The results of this modeling study of surface defects on graphene sensors is very important for understanding how to engineer advanced chemical vapor sensors for increased sensitivity and selectivity.