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Calculating pH of Weak Acid and Base Solutions

Calculations used to determine pH in solutions of weak acids or bases

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How Strong?

How Strong?

Credit: Bark
Source: http://www.flickr.com/photos/49503168860@N01/5518697538
License: CC BY-NC 3.0

How do you like your coffee? Some people like it strong enough to blow their eyes open (especially first thing in the morning) while others prefer their coffee to be a little tamer. Espresso stands prepare cups individually, so you can order a single shot (the least expensive) a double (very common order), a triple (for those who live a little more on the edge) or a “quad” (a quadruple shot that will definitely get your heart beating). For coffee lovers, the terms “strong” and “weak” can at least be semi-quantitated.

Why It Matters

  • Conversations about coffee strength tend to be vague and subjective. Yes, a double has more caffeine in it than a single, but we really can’t put numbers to the issue (analyzing caffeine content would be time-consuming and expensive, not to mention a waste of good coffee). When we talk about strong and weak acids and bases, we can put numbers to things, but there is still a little vagueness about our conversation.
  • Strong acids and bases are those that dissociate 100% in water. So there should be complete release of hydrogen ions for a strong acid and we see that with a few compounds. The pH of a 0.1 M solution of HCl will be around 1.1, so we can call that a strong acid. Strong bases are indicated by a high pH in solution. It makes sense that NaOH would give a pH of 13.
  • Credit: sylvar
    Source: http://www.flickr.com/photos/sylvar/5845762134/
    License: CC BY-NC 3.0

    Because strong acids are so reactive, they must be carefully transported in specialized vehicles [Figure2]


  • More interesting are the weak acids and bases. Acetic acid is considered a weak acid, giving a pH of 2.9 in water solution. Sodium acetate (the conjugate base of the acid) produces a pH of 8.4 and is a weak base. An equilibrium will exist among the undissociated acetic acid, the acetate anion, and the water in equilibrium with its H+ and OH-. So the acetate ion can capture a proton from the water, increasing the hydroxide ion concentration and the pH at the same time.
  • Watch a video about strong and weak acids and bases at the link below:


Show What You Know

Use the links below to answer the questions that follow.

  1. What is the base ionization constant
  2. Does a weak acid have a higher or lower pH than a strong acid?
  3. Take the quiz at the chem.wisc.edu web site and see how well you do.
  4. How do weak bases function?

Image Attributions

  1. [1]^ Credit: Bark; Source: http://www.flickr.com/photos/49503168860@N01/5518697538; License: CC BY-NC 3.0
  2. [2]^ Credit: sylvar; Source: http://www.flickr.com/photos/sylvar/5845762134/; License: CC BY-NC 3.0

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