When you describe how acidic or basic a solution is, you're describing the concentration of two of its ions. The first, hydronium (H3O+), forms when a hydrogen ion from water or a solute attaches itself to a water molecule. The second, hydroxide (OH-), forms when a solute dissociates into hydroxide or when a molecule of water loses a hydrogen ion. A solution's pH describes both the hydronium and the hydroxide concentration using a logarithmic scale.
Multiply the solution's pH by -1. For example, imagine a solution with a pH of 3.3--3.3 x -1 = -3.3.
Raise 10 to the power of the result--10^-3.3 = 0.00050118723, or approximately 5 x 10^-4. This is the concentration of hydronium ions, measured in moles per liter.
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Subtract 14 from the pH--3.3 - 14 = -10.7.
Raise 10 to the result--10^-10.7 = 1.995 x 10^-11, or approximately 2.0 x 10^-11. This is the concentration of hydroxide in the solution, measured in moles per liter.