Because of the importance of pK a prediction, and the difficulty of predicting pK a values, the SAMPL challenge organizers included a pK a prediction component in SAMPL6. Experimental macroscopic pK a s were collected for 24 drug like molecules using an established spectrophotometric technique limited to a pH range between two and twelve. As a part of follow up analysis, a few NMR experiments were performed to determine the microscopic pK a s of a select few molecules . Microscopic pK a s refer to an equilibrium resulting from removing a specific hydrogen from a molecule and macroscopic pK a s describe the process of removing any hydrogen or an overall change in charge state . All experimental data was kept secret from the public to allow participants in the challenge to make blind microscopic and macroscopic predictions for the 24 molecules. Specifically, there were three formats allowed for prediction submission: