Determining Ca 2+ Content by EDTA Titration (QUESTION AT BOTTOM) Steps: 1.Pipet

Question

Determining Ca2+ Content by EDTA Titration (QUESTION AT BOTTOM)

Steps: 1.Pipet 10.00 mL of unfltered seawater into a 250-mL Erlenmeyer flask, add 50.0 mL DI water.

2. Add 30 drops of 50% NaOH solution. Allow 5 minutes for magnesium hydroxide to precipitate. The precipitate may not be visible, it may be cloudy. Record your observation.

3. Weigh out ~0.1 g of solid hydroxynaphthol blue indicator. Add the indicator to your sample, swirl to dissolve.

4. Titrate the sample with the EDTA titrating solution to a blue-violet endpoint (not as deep as Part 2).

5. After you reach the indicator endpoint, let your sample sit for 5 minutes in order to allow any Ca(OH)2 that may have co-precipitated with the Mg(OH)2 to redissolve.

QUESTION: What is the chemical reaction that describes the re-dissolution of Ca(OH)2?

Explanation / Answer

The reaction is:

Ca(OH)2 (s) <-------> Ca++ (aq) + 2OH-

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