A primary goal of exoplanet observations has been to measure their atmospheric compositions in order to constrain their formation histories and chemistry. The key species to measure are those containing carbon and oxygen, as these are the two most abundant elements in the protoplanetary disk after hydrogen and helium and are crucial to the formation of giant planets. Precisely measuring C and O abundances has been difficult because the limited wavelength coverage and low resolution of space-based facilities means they only allow observation of a subset of the main C- and O-bearing species in planetary atmospheres. We propose a broad transmission spectrum survey using IGRINS on Gemini-S to precisely measure the molecular abundances of 12 high signal-to-noise transiting planets. The high resolution and broad spectral coverage of IGRINS will give us access to all of the primary C- and O-bearing molecules in exoplanet atmospheres, from which we will be able to derive precise abundances. Our targets span over an order of magnitude in mass, almost 2000 K in equilibrium temperature, and more than two orders of magnitude in age, which will allow us to investigate differences in planetary chemistry over a wide region of parameter space. Despite its potential as a tool for exoplanet science, there are no published results using IGRINS to analyze transiting exoplanet atmospheres. Therefore, as part of this proposal we will develop an open source data reduction pipeline for IGRINS transit observations.