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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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Practice Calculating pH of Weak Acid and Base Solutions

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How Strong?

### How Strong?

Credit: Bark
Source: http://www.flickr.com/photos/49503168860@N01/5518697538

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/

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

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?

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