Question

1. Compare the pHs of deionized water and tap water. Explain why the pH is not 7 and why the samples have different pHs.

2. Equal amounts of H+ and HO- are added to a buffer. What is the effect on the pH of the buffer? Explain.

3. How do you identify a suitable buffer system to maintain a pH of 3.0 in an aqueous solution?

Answer 1

The tap water is likely "hard," i.e., contains some dissolved mineral salts, most likely (primarily) calcium and magnesium carbonates and bicarbonates. The anions of these salts are slightly basic, consuming H+ ions and thereby raising the pH. Carbonates (with the obvious exception of alkali metal salts) are only sparingly soluble in water, while bicarbonates are typically very soluble. A couple of other subtle factors may affect the pH:

1. Lower atmospheric pressure encourages the evaporation of dissolved carbon dioxide from water. Since dissolved carbon dioxide exists in equilibrium with carbonic acid, it would tend to lower water pH. If you live in an area with comparatively low atmospheric pressure, your tap water might have a lower concentration of CO2, which would favor higher pH.

2. At higher temperatures, bicarbonate decomposes to water, carbonate, and carbon dioxide. Carbonates are relatively insoluble, and the consumption of bicarbonate should drive forward the dissociation of any dissolved carbonic acid, so a higher temperature should have the effect of lowering pH.

2)the additional OH