Apple juice and vinegar — Chemistry prompts about weak acids

Generating interest in weak acids

We started a unit this week applying equilibrium concepts to acid base chemistry. To motivate this discussion, we used prompts to engage students in the critical thinking skills of problem solving and lab design. Apple juice and vinegar were chosen as easy to engage weak acid systems. Acid strength and a relative danger is a common area for students to be confused about and this lab will allow for many opportunities to answer the question (are acids dangerous?) too.

The first prompt engages students in the relationship between K_a, pH and weak and strong acids.

The second prompt engages students in the relationship between these variables as well, but they will need to invent the science needed to get an extra unknown variable (in this case K_a).

I guide students through thinking at the end of the lab about the utility of doing a titration and finding the pK_a at the half-equivalence point.

Notes

Students figure out that they need to know the concentration of acetic acid in vinegar. While they can get a measurement of the H^+ concentration using the pH meter, if they do not know the initial concentration of the vinegar in the water, this will throw off their calculation.

So how can we measure the amount of acetic acid and H+? With a titration, and a pH measurement, we can find the equivalence point by seeking a place on the titration curve with a vertical asymptote. Using the amount of base added and the pH at the equivalence point, we can see both how much base was needed AND the amount of counter ion present at the equivalence point.

A HINT to use: What will the pH be at the equivalence point of a titration of acetic acid….

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I am a math and science teacher at a boarding school in Delaware.

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Posted in Advanced Chemistry

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