Chemistry
pH and pOH
pH measures how acidic or basic a solution is — pH = -log[H⁺]. pOH does the same for hydroxide: pOH = -log[OH⁻]. They always satisfy pH + pOH = 14 at 25°C.
How to Approach pH and pOH
Identify the substance
Strong acid/base (fully ionizes): use the given concentration directly. Weak acid/base: use Kₐ or K_b to find [H⁺] or [OH⁻].
Compute [H⁺] or [OH⁻]
For a strong acid like HCl: [H⁺] = concentration. For a weak acid, set up an ICE table and solve the equilibrium.
Apply the logarithm
pH = -log[H⁺]. Once you have pH, get pOH from pH + pOH = 14. Or compute pOH directly from [OH⁻] and use the relation.
Frequently Asked Questions
What's an acidic vs. basic pH?+
pH < 7 is acidic, pH = 7 is neutral, pH > 7 is basic. At 25°C, pure water has pH 7.
How do buffers work?+
A buffer is a weak acid + its conjugate base (or weak base + its conjugate acid). It resists pH change by neutralizing small amounts of added strong acid or base.
What's the Henderson-Hasselbalch equation?+
pH = pKₐ + log([A⁻]/[HA]). It's the fast way to compute the pH of a buffer solution.
Related Topics
More step-by-step guides in Chemistry and adjacent subjects.
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