Acids have low pHs and bases have high pHs. Water which is neutral has a pH of 7. Where do these numbers come from and what do they mean? This PowerPoint presentation explains how to calculate and interpret pH. The first half of the lesson shows how to approximate pH based on the exponent of the hydronium ion concentration. It is suitable for all levels of chemistry. The second half of the lesson shows how to calculate the pH for strong acids and bases of known concentration. It also shows how weaker acids and bases of known concentration can be dealt with using the equilibrium constants. It ends with an advanced topic - calculating the pH of buffers using the Henderson-Hasselbalch equation. Worksheets on this topic are available free of charge on my website at www.evanschemistrycorner.com.