[sucrose]

Let's say I have 2.1kg of sucrose dissolved in 1 litter of distilled water. I know the distilled water is 7 (in a vacuum where it doesn't come in contact with air). But what would the pH be once I dissolve the sucrose in the water?

[Arkcon]

You can generally use the published pKa of a molecule (or experimentally determine the pKa if the molecules is obscure) to determine the ph.  Sucrose is a tough one 'tho.  Ref: http://www.sciencedirect.com/science/article/pii/S0963996905001912

[sucrose]

Thank you. Since sucrose is problematic, let me use caffeine instead. I can see caffeine has a published pKa of 14.0 at 25C. Is there a formula I can plug this into, with the amount of caffeine and the amount of distilled water to get the pH?

[Borek]

A lot depends on the concentration, so choosing the right formula is not always simple and straightforward. Whole calculation process is explained here:

http://www.chembuddy.com/?left=pH-calculation&right=toc

Simpler approach, that ignores "why" (and effects of the solution ionic strength):

http://www.chembuddy.com/?left=BATE&right=pH-cheat-sheet