Need Chemistry Lab Help! On water correction...

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I have data for the pH of a buffer and the pH as a specific number of mL are added. And I have data for the pH of water and the pH as a specific number of mL are added. Now I need to make a water correction table so that the pKas on the graph are more easily determined and well defined. How do I do this from the data that I have? Do I just subtract the water data from the buffer data and then take the absolute value of that and graph it? Or what am I supposed to do? Our professor is not very clear because he basically expects us to already know everything. Well, I've never been asked to do a water correction table and graphs before so I'm not sure what to do. Any help is greatly appreciated!