Acid Rain and Buffers

Question

In the Midwest, the natural geologic formation are typically made of limestone ( CaCO3). The limestone in the Midwest provides the lakes and soils with a natural buffer. The questions beloiw highlights the benefits of natural buffers, or buffers in general.

Pure Water

a. What is the pH pf 1.000 L of pure water?

b. If you add strong acid to pure, unbuffered water, what magnitude of pH change do you expect? ( pH drops a little, a lot, not at all. )

c. A 200.00ml sample of 1.000M nitric acid was added to the water sample above. What is the resulting pH?

Answer 1

a). pH of pure water depends on the temperature. At 25ºC pH of pure water = 7. The volume (1.000 L) or any other volume does not affect the pH.

b). Adding a strong acid to unbuffered water will result in a large change in pH. The pH will fall drastically.

c). If 200.00 ml of 1.000 M nitric acid (HNO3, a strong acid), is added to 1.000 L of pure water, the pH can be calculated as follows:

moles HNO3 added = 200.00 ml x 1 L / 1000 ml x 1.000 mol / L = 0.2000 mols HNO3 added

Since it is a strong acid, this means 0.2000 moles H⁺ have been added

Total volume is now 1.2000 L, so [H⁺] = 0.2000 mol / 1.200 L = 0.1667 M

pH = -log [H+]

pH = -log 0.1667

pH = 0.778