Detailed Instructions: In this task you will be given a text passage from a trivia quiz. You should choose the category this question belongs to from these categories: History, Science, Social Science, Fine Arts, Literature. You can only choose one category, so if in doubt, please choose the closest match.
Q: One analogue of this quantity, designed for use in concentrated solutions, is equal to -19 for a certain mixture that contains antimony pentafluoride. One equation for this quantity, which may be derived by rearranging the defining equation for "K sub a", is called the Henderson- Hasselbalch equation. Under standard conditions, this quantity is half the self-ionization constant pKw. This quantity is defined as the negative logarithm of the concentration of hydronium ions in a solution. For 10 points, name this quantity which can be crudely measured using indicators like phenolphthalein, and which is 7 for neutral solutions and less for acids.
A:
Science